A Horde of Hoofbeats

My observations on how to manipulate the mount counter for this achievement:

1. CLASSES WITH CLASS MOUNTS
Some classes (Paladin, DK, Warlock) have their own class mounts which gives you additional points to your counter when you log onto them, the counter stays updated to the highest amount even if you log back to a class without class mounts.

Experiment & observation:

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• On my monk, the counter says 380 mounts.
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• I log paladin, the counter now adds +2 class mounts, it now shows 382 mounts.
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• I log back onto monk, the counter still shows 382.
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• On my monk, I have 3 mounts in my bag.
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• I learn mount #1 - the counter does not move.
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• I learn mount #2 - the counter does not move.
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• I learn mount #3 - the counter updates to 383.
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• I log paladin, 2 class mounts are added, updating the counter to 385.
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NOTE for Argent tournament: The paladin can buy one additional mount at the Argent tournament [item=47179], increasing class mount advantage from 2 to 3.

Summary: Relog to characters with class mounts to update the counter to the highest possible number. If you have a paladin, do argent tournament on it to get an additional class mount.

2. PROFESSION MOUNTS
You do not need to have mount-giving professions on one character to maximize counter.

Experiment & observation

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• On my monk, I have engineering with 2 mounts.
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• On my paladin, I have tailoring with 4 carpet mounts.
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• On my paladin, I learn engineering
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• In practice, I can now use 2 additional engineering mounts on my paladin
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• HOWEVER the mount counter did not increase.
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Summary: I am not 100% sure about this, but my theory is as follows: As long as you have at least one character on your account with a unique-mount profession (Engineering, Tailoring, Leatherworking) and have the mounts learned, they should contribute to the global counter. Putting tailoring and engineering/leatherworking on a single character to maximize the counter is not necessary.


3. LEGION ORDER-HALL MOUNTS
I can confirm with 100% certainty, that these mounts contribute to the global mount-counter even if other classes cannot use them.

Experiment & observation

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• I completed the order-hall mount quest on my mage.
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• The counter did not increase, because my mage has less mounts than some of my other characters.
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• I log paladin (character with most mounts), the counter is updated by +1.
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• Edit: completed order-hall mount on 2 more characters - they each added +1 to the counter on relogging to my paladin
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Summary: Acquire order hall mounts on all possible classes (except druid) to maximize counter.


4. OTHER FACTION'S ALLIED-RACE MOUNTS
I confirm that completing the other faction's allied-race scenarios increases the counter on your main faction.

Experiment & observation

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• I completed the Nightborne and Highmountain scenario on my Horde Druid
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• On relogging to my Alliance Paladin, the counter increase by +1 each time
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5. TL;DR VERSION

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• The ‘usable-by-a-single-character’ clause seems to be misleading.
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• Participate in the Argent Tournament on your paladin to increase class-mount advantage from 2 to 3.
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• You can have the profession-specific mounts on any character. Don't bother putting tailoring with engineering/leatherworking on a single character to maximize counter.
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• Complete the order-hall mount quests on each class, since they all contribute individually to the global counter (except probably druid).
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• Complete all allied race quests if possible, mounts from the other faction count.
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These are my observations for gaming the mount counter in the achievement and getting it as efficiently as possible. Good luck!

Hello guys
While there are many comments and threads about mount collecting achievements, I would like to make clear a few things since they are still debatable.

As of patch 8.1 there is a chain of 10 achievements starting by collecting 10 mounts to the last one which is gained by collecting 400 mounts
Note that currently there are 497 obtainable mounts (that can count toward the achievement counter (or as blizzard says: 'Usable by a single character' but this is not a true statement as we will see below)

Just for reference these achievements are listed below:
1. Stable Keeper
2. Filling Up The Barn
3. Leading the Cavalry that gives you [item=44178]
4. Mountain o' Mounts / Mountain o' Mounts that gives you [item=44843] / [item=44842]
5. We're Going to Need More Saddles / We're Going to Need More Saddles that gives you [item=91802]
6. Mount Parade / Mount Parade that gives you [item=98259] / [item=98104]
7. Mountacular / Mountacular that gives you [item=69226]
8. Lord of the Reins / Lord of the Reins that gives you Heavenly Azure Cloud Serpent
9. No Stable Big Enough / No Stable Big Enough that gives you [item=137614]
10. A Horde of Hoofbeats / A Horde of Hoofbeats that gives you [item=163981]

Before we begin, lets make clear that the number that appears in your collection tab is not the one that matters for the achievement because this number represents the total number of mounts you own on YOUR ACCOUNT
There are many mounts (mostly main cities/racial mounts) that have a counterpart in the opposite faction.
For example if you obtain the [item=13086] in an alliance toon you will automatically obtain [item=46102] in your horde toons
That means that in your collection tab will be added 2 mounts but actually in your achievement tracker will be added 1 mount (since a specific character can use 1 additional mount, not 2).

First of all, all mounts are acount-wide except [item=54465] but some of them are not counting on all of your toons
Below I will list the mounts that can confuse you since some of them are counting on all of your toons but some of them aren't

CONTROVERSIAL MOUNTS

-PROFESSION MOUNTS
There are some profession mounts that can craft/learn/ride only while you own a specific profession BUT ONCE YOU LEARN THEM THEY ARE COUNTING ON ALL OF YOUR TOONS. That mean that it doesnt mater if you learn them in you main/alt and you can even delete this profession once you learn these mounts.
These mounts are:
Tailoring: Flying Carpet , [item=44558] , [item=54797] , [item=115363]
Leatherworking: [item=108883] , [item=129962]

-LEGION CLASS HALL MOUNTS
ALL LEGION CLASS HALL MOUNTS ARE COUNTING ON ALL OF YOUR TOONS ONCE YOU LEARN THEM NO MATTER IF THEY CAN USE THEM
That means that every new class hall mount that you learn in a new class (using your alts) is counting as a '+1' in your achievement.
You have a potential of +21 Mounts (toward the achievement) if you collect each one of the class hall mounts (pretty challenging tbh)
Priest: High Priest's Lightsworn Seeker
Mage: Archmage's Prismatic Disc
Death Knight: [item=142231]
Monk: [item=142225]
Warrior: [item=142232]
Shaman: [item=143489]
Demon Hunter: Slayer's Felbroken Shrieker
Hunter: [item=142228] , [item=142226] and [item=142227]
Rogue: [item=143492] , [item=143490] , [item=143491] and [spell=240982]
Paladin: [item=143502] , [item=143503] , [item=143505] and [item=143504]
Warlock: Netherlord's Chaotic Wrathsteed , Netherlord's Accursed Wrathsteed and Netherlord's Brimstone Wrathsteed

-CLASS MOUNTS ***NOT TO BE CONFUSED WITH LEGION CLASS HALL MOUNTS***
SOME CLASS MOUNTS ARE COUNTING ON ALL OF YOUR TOONS ONCE YOU LEARN THEM NO MATTER IF THEY CAN USE THEM BUT THERE ARE SOME OF THEM THAT AREN'T)
Below I will list/show you which are these mounts and how they are working:

MOUNTS THAT ARE COUNTING ON ALL OF YOUR TOONS ONCE YOU LEARN THEM
Warlock: Felsteed , Dreadsteed
Death Knight: Acherus Deathcharger
Paladin: [item=47179] (This can be bought in cost of 100 [currency=241] )

MOUNTS THAT ARE ***NOT*** COUNTING ON ALL OF YOUR TOONS
Paladin: Summon Exarch's Elekk , Summon Great Exarch's Elekk
Demon Hunter: Felsaber

As we can see, Paladins got 2 mounts that are counting only on them while Demon Hunter's got only 1
That makes Paladins the holders of the highest possible number in the game.
Theorytically speaking, you dont even need Demon Hunter's Felsaber as you can see.

-VASHJ'IR MOUNT ***NOT ACCOUNT WIDE***
[item=54465]
THIS MOUNT NEED TO BE OBTAINED ON EACH TOON INDIVIDUALLY.
Its CRUSIAL to obtain this mount on your PALADIN in order to get the highest possible number because if you don't, your paladin will got 1 less mount than the higher possible number.
In order to Obtain this mount you need to complete a short questline starting with Call of Duty and ending with The Abyssal Ride
If the starting quest is not available for you, make sure to complete the attunement quests of Broken Isles / Argus / Kul TIras or Zandalar, then the quest will be available for you.

Keep in mind that the achievement tracker counts each time you log in with a toon.
For example if you log in with your Paladin and your Paladin got the highest number of mounts (lets say: 382), then when you log in in your main the counter will still say '382', even if you get one more mount the tracker will not change but the next time you will log in with your paladin the tracker will say '383'
You can check each toon's mounts in Achievements tab -> Statistics -> Character -> Mounts Owned

-HOW TO GET THE HIGHEST POSSIBLE NUMBERS OF MOUNTS
-Have a Paladin and level him at least to level 80 (that will grants you the Summon Exarch's Elekk , Summon Great Exarch's Elekk and gives you the accessibility to obtain the [item=54465]
-Get [item=54465] on your Paladin
-Learn Leatherworking and Tailoring and Create/Learn the 6 mounts we mentioned above (after you learn them you are free to delete your profession or whatever you want)
-Have a Warlock at level 40 ( at level 40 you have learned both Felsteed and Dreadsteed )
-Have a Death Knight (no matter what level your DK will be, you only need his Acherus Deathcharger )
*Dont forget [item=47179] that can be obtained only by Paladin but I scarcely listed it here since we can consider it as a normal mount

At this Point your Paladin will be +2 mounts that anyother toon (if this toon got [item=54465] ) otherwise will be +3.
You dont have to log in to your Paladin everytime you obtain a new mount, Just make sure to do it when you are 2-3 mounts away from an achievement

Good Luck and Have a Nice Hunting :)

They could add new achievement and mount for 450 etc.... i think collectors would appreciate it, myself included