Are there any rules anywhere that give ideas for magi that want familiars other than simple animals? Like if one wanted a "land octopus" or a "sentient shadow"?
I can always figure something out, but I don't want to reinvent the wheel if it's already out there somewhere...
If it's a physical creature (not a human), associated with the magic realm, have fun.
Mind you, to bind that Might 30 size +2 Griffin, you'll need a lab total of 65+ but other than that, have fun.
Binding an immaterial spirit requires a virtue (but carries extra bonuses IIRC). See The Mysteries, Revised Edition.
Binding a faerie requires Faerie Magic (again, IIRC), and comes with a few complications. See Houses of Hermes: MYstery Cults (Merinita chapter).
Binding a Demon... is a really bad idea, but I don't think it actually requires any special virtues. Would probably give you a nice flaw though.
Binding an Angel... might be possible for magi with holy magic, but seems horribly Arrogant and so probably wouldn't be virtuous.
The important factors are game-balance and believability/acceptability for your Troupe. Such creatures probably are not found in the local petshop so - how did you find one?
Otoh, if that's the character concept, and it's OK w/ the SG/Troupe, no reason not to - it's certainly in genre (if at the higher-fantasy end of the spectrum).
I want to say there is a Virtue (forget which source) that allows "exceptional familiar" (or words to that effect) without the need for an in-saga effort - a "fast pass" to the front of the line, as it were. (I've never used it - any unusual familiar worth having is worth a story imo/ims.)
A nice Flaw, and extra Virtue/powers/etc: Hell is really good at offering a (worldly) Upside, to tempt those Foolish Mortals...
I might allow (with research, as a breakthrough) something mutual, a binding-to-one-another; it will quite likely leave the Angel as the more-influential partner in the pairing, and offhand I suspect it would move the PC substantially toward NPC-dom.
There are also the Rules for faerie familiars in HoH:MC. Basically Merinita can bind any living thing that is inherently faerie in nature. They leave what that means pretty wide open. If it has a fae based might score defiantly. But book specifically mentions the possibility of binding people or animals, who lack might scores but are otherwise closely associated with faeries.