Pest Control

If it can be found in Northrend, that will be listed. Otherwise, any Outland location. Vanilla WoW locations are only listed if it can only be found there.
Northrend:
If you are starting off in Dalaran, take care of Underbelly Rat in the sewers before you leave town. Then, I suggest the following path.
Fly into Howling Fjord and get
Devouring Maggot- Howling Fjord, Utgarde Catacombs
Fjord Rat- Howling Fjord
Roach- Howling Fjord
Spider- Howling Fjord , Ahn'kahet: The Old Kingdom, Azjol-Nerub, Drak'Tharon Keep, Naxxramas, The Culling of Stratholme, etc.
While you are in the Howling Fjord, go /love these guys For the To All The Squirrels Who Shared My Life achievement.
Fjord Turkey - Howling Fjord
Fjord Penguin - Howling Fjord coasts
Scalawag Frog - Howling Fjord, Scalawag Point

Then , on your way to Zul'Drak to:
kill Zul'Drak Rat,
and love Arctic Hare
EDIT: Reports of Snake in GunDrak
stop by Grizzly Hills and /love:
Mountain Skunk - Grizzly Hills
Grizzly Squirrel - Grizzly Hills
EDIT: Several reports of a pack of deer and Mice in this zone near Venture Bay, I have not confirmed this though.

Continue north to Storm Peaks and kill
Gold Beetle- Halls of Stone, Halls of Lightning *no fighting necessary- see comment at bottom*

Now head west into Icecrown to /love
Glacier Penguin

Now head Southwest to Sholozar Basin and show some love to
Sholozar Tickbird, there is a flock right in the Nesingwary camp.

Now, continue south to Borean Tundra and love
Borean Marmot - Borean Tundra
Tundra Penguin - Borean Tundra coast
before heading into the Nexus to kill
Crystal Spider- The Nexus, Oculus *no fighting necessary- see comment at bottom*

If you are running Naxx anyway, these pests can be handled during your run, otherwise alternate locations are listed.
Larva- Naxxramas, Ghostlands
Maggot- Naxxramas, Ghostlands
Rat- Naxxramas, The Culling of Stratholme, Terokkar Forest

Now it is time to head to Outland:

Outland
In Zangermarsh, kill
Snake- Zangarmarsh EDIT: My understanding is that this critter can be found in Westfall. You have to go there anyway for the Mouse, so you might cut this zone out of your path. EDIT #2: Reports if this critter also in GunDrak- might be worth checking out when you are up that way.

Fly into Hellfire Peninsula and snuff
Adder- Hellfire Peninsula
Scorpion- Hellfire Peninsula

Old World
Use the Portal to get to the old world because that will put you close to:

Moccasin- Swamp of Sorrows
While you are in the swamp anyway, /love Huge Toad

Then, head into the Searing Gorge to:
kill Fire Beetle- Searing Gorge\
and love Lava Crab

Finally, go to Westfall for this pest.
Mouse- Westfall

If I have left anything out, please let me know and I will change it.

Edited and sorted by area, which seems easier to me. There are a total of 4 in Naxx if you are running it anyway. However, all of these can be found in other places. The other 2 that are found in dungeouns should be close to the front door, so you should not have to fight any elites.

If you want to do [To All The Squirrels Who Shared My Life] and [Pest Control] together, both and in some sort of order:

Northrend:
Kill: Devouring Maggot - Howling Fjord 57,55 in the Utgarde Catacombs on the bottom pit with all the ghouls
/love: Fjord Turkey - Howling Fjord 67,67
/love: Fjord Penguin - Howling Fjord coasts around 21,61 and 66,83
/love: Scalawag Frog - Howling Fjord 36,80
Kill: Fjord Rat - Howling Fjord
Kill: Roach - Howling Fjord 36,11 and several other places / instances
/love: Mountain Skunk - Grizzly Hills
/love: Grizzly Squirrel - Grizzly Hills
Kill: Zul'Drak Rat - Zul'Drak 45,63 around the Amphitheater
/love: Arctic Hare - Zul'Drak 56,78 e.g., Dragonblight, Borean Tundra
Kill: Gold Beetle - Halls of Lightning just behind entrance, Halls of Stone
/love: Glacier Penguin Icecrown 69,20
/love: Sholazar Tickbird - Sholazar Basin 27,60
/love: Borean Marmot - Borean Tundra
/love: Tundra Penguin - Borean Tundra at several coast spots
Kill: Crystal Spider - Nexus around Keristrasza, Oculus
Kill: Underbelly Rat - Dalaran Sewers

Naxxramas: (Gotta go there anyway, assuming you don't want to travel to Ghostlands)
Kill: Larva - Naxxramas Plague Wing, Ghostlands
Kill: Rat - Naxxramas, Arathi Basin (Farm if Horde controlled) and a lot of other places
Kill: Spider - Naxxramas entrance, several LK 5 man instances and several other places
Kill: Maggot - Ghostlands, Naxxramas entrance, Underbog

Outland:
Kill: Adder - Hellfire Peninsula, Nagrand, Durotar, Barrens
Kill: Scorpion - Hellfire Peninsula, AQ20, AQ40

Eastern Kingdoms:
Kill: Mouse - Westfall
Kill: Snake - Westfall and several other places / instances
Kill: Moccasin - Swamp of Sorrows
/love: Huge Toad - Swamp of Sorrows
Kill: Fire Beetle - Searing Gorge, Burning Steppes, Shattered Halls
/love: Lava Crab - Searing Gorge, Burning Steppes

I'm listing the easiest places to find each critter.

• 
  
• Adder - Durotar, The Barrens, Hellfire Peninsula, Nagrand
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• Crystal Spider - The Nexus and The Oculus
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• Devouring Maggot - Howling Fjord (Utgarde Catacombs)
• 
  
• Fire Beetle - Searing Gorge, Burning Steppes and The Shattered Halls
• 
  
• Fjord Rat - Howling Fjord
• 
  
• Gold Beetle - Halls of Lightning and Halls of Stone
• 
  
• Larva - Ghostlands and Naxxramas
• 
  
• Maggot - Ghostlands, Naxxramas and The Underbog
• 
  
• Moccasin - Swamp of Sorrows
• 
  
• Mouse - Westfall
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• Rat - Nagrand, Ghostlands, Stranglethorn Vale (Booty Bay), Drak'Tharon Keep, The Culling of Stratholme
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• Roach - Howling Fjord, Ahn'Kahet, Azjol-Nerub, The Culling of Stratholme
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• Scorpion - Hellfire Peninsula, AQ20 and AQ40
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• Snake - Zangarmarsh, Ghostlands, Westfall
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• Spider - Howling Fjord, Ghostlands, Ahn'kahet, Azjol-Nerub, Drak'Tharon Keep, Naxxramas, The Culling of Stratholme
• 
  
• Underbelly Rat - Dalaran Sewers
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• Zul'Drak Rat - Zul'Drak
•