few pets i found easy to find and easier to level..
( you said you have soem sub 20 pets already .. so keeping that in mind )
Crimson geode - Deepholm (lvl 23 ish )
takes damage first 2 rounds .. 3rd round pretty much one shots everything .( i will let u figure out how
Emrand proto whelp - ( scholzar basin )
decent damage and heals like god , you can also swap one heal to boost damage potential .
you may find it hard to farm a rare one , i would settle for uncommon if unlucky with rares.
thanks to battle stones , you can upgrade later.
and most fav one is spawn of onyxia
low health , but excellent damage and superb healing ..
In fact , i could solo 3 wild pets , 2 lvl higher than my onyxia , in wild pet battles.
made leveling lowbies much much easier and faster.