Treats are items that you can use in your creature cage to feed your familiars and increase one of their six stats by varying amounts. You can get them after a battle, from side quests, in chests, or you can purchase them at vendors.

Feeding your familiars will increase their fullness meter, up to a maximum of ten. The only penalty to this is that you won't be able to feed them more until they get hungry again. With every fight, the meters decrease by one.

There are six types of treats, one for every stat. Each genus has a favorite treat, which will boost the stat gain it gets from eating. You can check Chapter 5 of your Wizard's Companion to determine the genus of your familiar if it isn't readily apparent.

Increases Attack
Favored by: Dracones, Milites

Increases Defense
Favored by: Automata, Flora

Increases Magical Attack
Favored by: Arcana, Mortui, Nymphae

Increases Magical Defense
Favored by: Daemonia, Vermes

Increases Accuracy
Favored by: Aquatica, Minima, Nobilia

Increases Evasion
Favored by: Aves, Bestia