Mawsworn Soulhunter

You can obtain this mount rarely (1.5%ish according to wowhead DB) on the ELITE Gorged Shadehound inside The Maw in "The Beastwarrens" part. He's part of an event called Hunt : Shadehounds in which you will have 6 packs of 3 adds to kill and then the elite becomes attackable. After all 6 packs are dead, he runs into the cave behind him and becomes attackable. Come with a little group because he has 559k health and he hits for a bit (2 or 3 people should be enough if you have a tank or pets). At 50%, he runs even deeper in the cave. You can click the chains to make you jump through the hole not taking any fall damage.

I personally only have seen the event on coords 53 78 (https://ibb.co/2tp2S0F).

/way 53 78 Gorged Shadehound Event

This mount has the particularity to be ridable in The Maw (the only two others mounts which are ridable inside of The Maw are [item=184166] that you can obtain by finishing "Twisting Corridors" wing in Torghast at Layer 8 and [item=184168] (unlocked at 9.0.5) that you can "craft" doing a complete puzzle in the Maw, more information here).

You DO can access this part from the beginning but you will have to deal with [spell=336972] that will periodically fear you for a few seconds and deal HUGE amount of damage (60-70% of your life).

You will not have this deadly debuff anymore in "The Beastwarrens" by using [item=182955] that you will receive doing the quest Rule 6: Concealment is Everything that you will get once you reach "Ambivalent" reputation level with Ve'nari. HOWEVER, you DO can drop the mount even without having reached Ambivalent (and quest) reputation. But you will have to deal with the fears and big damage.

This elite is killable on a hunt-bi-weekly rotation, like 8.3 invasions (and their rares). so killable ONE time in 3.5 days and then not killable for 10.5 days (others hunts during that time). Killing him will also grant you 525 reputation and one complete level with the Eye of the Jailer.

Knowing this, you DO CAN farm it on multiple chars who just unlocked The Maw.

TL:DR : To get this mount : Reach "Ambivalent" reputation level with Ve'nari, do the Rule 6 quest, unlock Beastwarrens part of the Maw, do the event and kill Gorged Shadehound bi-weekly, it means killable ONE time in 3.5 days and then not killable for 10.5 days (others hunts during that time).
You however can kill the Gorged Shadehound and drop the mount without having reached "Ambivalent" reputation level, but then get a friend healer or lot of food to overstep [spell=336972]

Unfortunately, Summon Random Favorite Mount does not understand the mount restrictions in the Maw, and will not be smart enough to prioritize this mount when you're there. You can use this macro to prioritize it in the Maw and fall back to Summon Random Favorite Mount otherwise:

#showtooltip Arboreal Gulper
/run if GetZoneText() == "The Maw" then C_MountJournal.SummonByID(1304) else C_MountJournal.SummonByID(0) end

Change "Arboreal Gulper" to whichever mount you'd like displayed on the macro's icon.

never thought it was rare cos got it from 1st try. Looks like fortune finally smiled to me. But...4 weeks of ZERO loot in heroic Castle was a compensation. And probably even more >.<.

Actually I dont get a idea to make a mount which on weekly rotation with other events (so you can see it only once per month) with drop 1.5? Blizzard rly want players not only observe "hell" in this game. But also to feel it and suffer. Pretty sure 8th level of Torgast wouldnt be ez. I wasted like 1.5h for first level and last boss was over 3 million hp. Almost sure with gear below 220 it would be close to imposible.