Mounts, animal companions, and familiars... Combined?

[lbxzero]

Right now, I am working on a cavalier, but I got some weird idea. If I later took a class that permits having a familiar, can the cavalier's mount be both? Would that require any special feats if possible?

[blahpers]

No, you can only combine animal companions, and then only if the companion is valid for both abilities that grant the companion. So a cavalier/druid could have a single full-level murderhorse or a somewhat weaker horse and a somewhat weaker roc. In such cases, Boon Companion and whatever feat lets non-cavalier levels stack for mount are your friends.