the foxhound in the tutorial (camp at the beginning) has a chance of about 60% to get catched :D but its a newbiepet
2nd highest chance is in Rainbow Valley. you can enter there with lvl 30+ and the pets there have a chance of about 15% ( = about 1 pet in 15 kills )
the normal mobs have a low chance, so good luck there ;)
lords have a higher chance than normal i think, but cause of respawn time and their dmg+hp its easier to farm normal mobs
i farmed for void biter about 200 mobs and got only 2 :O
but the pets in rainbow valley are better than normal mobs and are meldable too