Here's a link to some info.
http://forum.us.runesofmagic.com/showthread.php?t=44326
It's not complete but will give you overview. Basically the type of stats given are based on elemental type of pet(water, earth, dark, light, wind, fire) and the pet egg rarity. Simply put you want a Holy pet egg. It will give you the greatest amount of stats. The other thing as you mentioned is aptitude. A pet with 100 apt gives more stat bonuses then a pet with 78. So leveling pet aptitude to 100 and keeping loyalty (this also has effect on stat bonus)at 100 are a must for getting the most out of your pet(not counting spending 200 million gold to level to 50 for all the passive bonuses).