All items have feed power.
Feeding your pets is built around your fame, it costs you fame each time you wish to feed your pet
Their is 5 levels of pets. Common, uncommon, rare, legendary , divine.
You need to fuse 2 commons to make a uncommon etc etc.
Its best to max your pets level before you fuse them for max strength.
Go to rotmg wiki pets to learn more about it.
The pets are really strong. At max rank can heal you for 90 hp and 45 mana every so often. If your pet has electric too any enemy that passes through your pet gets paralyzed. Pretty sick stuff.