Also, consider if you want it as an NPC, or how central this familiar will be. We have a shaman in our group, and I drew up his familiar as an NPC just to feed hooks and info to the group, because I wanted some of the plot to be more obvious.
We have a lot of characters in our group, so I didn't want another thing I had to track in combat as the GM. So it just doesn't enter combat, really. The spirit possesses any nearby dog when it shows up, so it only costs a fate point for the player to have the spirit show up if he needs it in a hurry, somewhere that a dog is unlikely to be. (Otherwise, it's a simple ritual to summon the spirit if time is not an issue).
However, our current story arc hinges on an outsider that has been bound away eons ago by native american magic users. Turns out, in a different time line, native american culture won out: they were more numerous and developed when the Europeans arrived, they went to war, won, conquered the world in time, and went forward past our modern day in a society based on open use of magic, led by shamans. However, the tribes never reached true peace, and one of them eventually called in an outsider that moves through time so that his tribe could eliminate other tribes before they ever started...and it all went to pot, some shamans went into the past to imprison the thing, ending up altering the time-line so drastically that our time-line happened instead.
Anyways, so the familiar spirit happens to know all of this because he was brought back from the alternate future when the shamans went to our own past... see this gets confusing, stupid time travel... and I use him frequently as a way into plot hooks. He likes to say "WE HAVE TO BE AT THE CORNER OF 5TH and MAIN TODAY AT 3:47PM AND I CAN'T TELL YOU WHY OR YOUR MIND WILL BREAK AND YOUR MAGIC WILL DARKEN!!" or the like.