I wrote an adventure once for my home campaign where the players each ran one of the PCs' animal companions, familiars, or pets. For that one adventure only, the 5 players ran a raven familiar, eagle animal companion, timber wolf animal companion, dire wolf pet, and the 70-year-old human NPC who helped care for them when he and the PCs were out in the field. They had to find out what happened to the PCs (who had all been simultaneously turned to stone in their headquarters) and find a way to undo it. It was an interesting change of pace.
Johnathan