I listed and sorted the locations for the both following Achievements, I hope this will help you: [To All The Squirrels Who Shared My Life] and [Pest Control]
All except the Larva in Naxxramas are soloable, but for that you can travel to Ghostlands
Dalaran:
Slay: Underbelly Rat
Dragonblight:
Love: Arctic Hare
Naxxramas:
Slay: Larva - Ghostlands, Naxxramas Plague Wing
Slay: Rat - entrance
Slay: Spider - entrance
Slay: Maggot - entrance
Howling Fjord:
Slay: Devouring Maggot - Howling Fjord 55,55 in Utgarde Catacombs where the ghouls are
Love: Fjord Turkey
Love: Fjord Penguin
Love: Scalawag Frog
Slay: Fjord Rat
Slay: Roach - 33,11
Grizzly Hills:
Love: Mountain Skunk
Love: Grizzly Squirrel
Slay: Zul'Drak Rat
Storm Peaks:
Slay: Gold Beetle- Halls of Lightning at the entrance
Icecrown:
Love: Glacier Penguin
Sholazar Basin:
Love: Sholazar Tickbird - 27,60
Borean Tundra
Love: Borean Marmot
Love: Tundra Penguin
Slay: Crystal Spider - The Nexus around Keristrasza
Outland Hellfire Peninsula:
Slay: Adder
Slay: Scorpion
Eastern Kingdoms:
Slay: Fire Beetle - Searing Gorge
Love: Lava Crab - Searing Gorge
Slay: Mouse - Westfall
Slay: Snake - Westfall
Slay: Moccasin - Swamp of Sorrows
Love: Huge Toad - Swamp of Sorrows