1 I was of the opinion that Arf wasn't so much summoned as created from whole cloth... I seem to remember reading something somewhere about that, but durned if I can find it now.  So take it with a huge grain of salt.

2 I'm building this on something Amy said to Yuuno when he first met Aria and Lotte. Amy said that their base was cats.

3 So how does Yuuno fit into this scheme, then?

4 Yuuno isn't a familiar.

5 Yuuno is a mage who knows how to turn into a ferret. Among other things, a familiar needs to run off of someone else's magic power, and he doesn't have ferret ears.

I do not think familiars are entirely biological. If this were the case, something that happens with Arf between A's and StrikerS would seem unwise and dubious to me.

My understanding is that Familiars are fabricated from whole cloth, at least as far as not being based on a creature already living. I do think it is quite possible that tissue is involved. Precia supposedly had a talent for it. By the rules of magical aptitude inheritence in the setting, this is apparently the reason Fate was able to create Arf at such a young age. (Precia is also where Fate got her lightning affinity, IIRC.) There are also some spoilers for StrikerS that might settle the issue.

Not like Me and Steven haven't had different interpretations of the same events before. Rule of Cool, and other such things.

6 Wonderduck: Could that have been the Manga?

7 We don't have a lot of examples and they're not wholly consistent.

We know that Aruf was a canine of some type that Fate turned into her familiar. (I was under the impression that it was injured/dying somehow, but I don't remember why.) So we know that's possible, and quite likely the source of most of the animal-type familiars. I do get the impression that the personality is not retained in that conversion - Aruf is someone that Fate created, not that Fate rescued.

Supposedly Precia, at one time, had a mountain-lion familiar. That's Linith, who we see in Fate's flashback.

Zafira's a special case, as a creation of the Book of Darkness.

Fate herself is a special case. Familiar creation was, if memory serves, Precia's area of specialty, so it's not going too far to assume that some of that knowledge went into her creation. Sure, Fate's essentially normal, not "Precia's familiar", but as an "artificial mage" she's definitely in a special category. (She's not the only one - there's some of these in Strikers too.)

And you can make devices that work like familiars, too, though it's kind of ancient Belkan / not done much. Rein Zwei from Strikers by way of example.

8 Pat, I haven't read the manga, but it's certainly possible that I picked it up by reading someone/thing on the 'net that HAD read it...