Your animal companion can become your familiar and vice versa.
Prerequisits: Must posses the Class Abilities Animal Companion and Familiar
Benefit: When you summon a familiar and do have an animal companion already or you have a familiar and may chose an animal companion, you can apply both ability-templates to the same animal.
The restrictions and abilities for both types applies (although the animal companion loses the ability to learn tricks, obviously).
The animal will only receive the bonuses and abilities granted by the separate classes, i.e. a 3rd level wizard 4th level driud Familiar Companion go an intelligience of 7, +4 natural Armor, +2 bonus HD, +1 Str/Dex adjustment, link, share spells, evasion, alertness, improved evasion, empathic link and deliver touch spells.
Normal: Animal companions can not become familiars.