All Radscorpions, Mirelurks, etc. count toward this achievement. You really shouldn't have any trouble getting this if you do all the side-quests and main quests. If for some reason you still don't have it toward the end of the game, wander the Wasteland and kill scorpions as they show up. Deathclaw Sanctuary is a good hunting ground as well.
Note for PC Users: You shouldn't need extra help for this either, but use "player.placeatme 00073E07 X" to spawn a dog, again with X being the number you want to spawn.