Common familiars in past games, were usually retained for Rangers, Druids, and Mages.
Mages could use them to aid them in spell casting, protection of an area (depending on the size), or to retrieve things from areas they could not go. Something like a cat, would be able to go into obvious places we could not fit.
Or a fairy dragon, something like Carkass, could aid in combat, by spell use.
Familars are generally treated in such away by intelligence, and all relationships between master/servant were considered very much as equals. So much so, that if the familar died, the bond between them wa so great, it causes pemanent loss of skills.
The talking features, I still think are great, this kind of spills into living items (talking swords), and dabbles into a couple other areas as well. This is a great idea.
Rangers, were usually aided in combat, but a familiar in that sense, is not the traditional type, it could be a magical mount. Think of Gandalf\'s horse, Shadowfax... but this types of mounts have also been reserved for the Paladin class. Rangers, could have something like a pixie, gnome, or other small woodland creature, like a racoon, squirrel, or otherwise.
Mage familairs were usually magically summoned. Where a ranger formed his bond through time naturally through gaining the animals trust. Which falls into the \'animal trainer\' skillset.
This could get extremely complicated. But is a traditional part of any good RPG.
** EDIT ** The most famous familiar I can think of, was the mechanical owl, from \'Clash of the Titans\'. What a horribly cheesy movie, but was it ever cool when I was younger.
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