Sorry... I wrote the wrong response there.. the uncooked meat from mobs may not give full happiness... here is a rundown.
Monitor the Combat Log of your chat box to see how much happiness your pet gets from the food. If the level of the food is 35-25 levels lower then the pet it gains 8 Happiness per tick, If the level of the food is level 25-15 it gains 17 happiness, if the level of the food is 15 levels lower or higher it gains 35 happiness.
Food for pets can be purchased from vendors. Vendors typically only sell one type of food, and finding the specific vendor(s) your pet needs can be problematic for pets with restrictive diets.
If your pet eats fish, cooked fish is amazingly cheap at fishing-related vendors. For example, a full stack of Longjaw Mud Snapper is less than 1 silver and pets up to 36 will eat it for middle (17 per tick) happiness gain.
You can take a pet that eats meat and hunt mobs that drop meat. These pets are the easiest to maintain.
Pets which eat meat will eat the meat which is not food to you (no health restored tooltip), uncooked (for example, Mystery Meat). It is frequently top tier, happiness wise, if the mobs are your pet's level.
You can take up Fishing and choose a pet that will eat fish. This will save money as compared to buying food, but fishing can take a lot of time, so there's a trade off.
Pets that eat bread can eat food conjured by mages.
If your pet has a DoT debuff (from a poison or bleed effect), make sure to use Mend Pet or wait for the DoT effect to end before feeding it. Otherwise, the next tick of damage will cancel the Feed effect.
*Maybe this should be in the FaQ Aeth?*
