Chemlak |
Pretty much just common sense.
Some GMs might want to require that certain animals need to be persuaded to wear some items, though, since animals are unlikely to understand the "need" to wear items. I doubt a dog would complain too much about a collar, but convincing it to wear a headband is something else entirely, not to mention how a bear would respond.
asthyril |
in PFS animal companions and normal familiars can only use neck slot and barding/armor. improved familiars that are humanoid in shape get normal character item slots.
other than that outside of PFS it is whatever makes sense to your GM. i think creatures should be able to wear a ring on a tail or something similar, maybe head slot items or capes.
Weirdo |
Also ask your GM about modifying certain items to better fit a companion. For example, a cloak of resistance might get in a cat's way, but with a quick tailoring job you could turn it into a much more comfortable vest of resistance.
A familiar should be smart enough to understand the benefit if these items. An animal companion might need a bit of convincing, but a Handle Animal check or Speak with Animal really ought to do it. They might not see the need for that vest, but they do trust and respect the person telling them to wear it.