Story!
FWIW:
Nothing in the Code speaks of human vs non-human.
In some cases, it would be very difficult to tell the difference.
The Rhine Tribunal would probably rule that a Hermetic Magus with Magic Might was a legitimate magus, to support their extra-sigil custom. House Bjornaer would probably not like anything that suggested that a magus in Heartbeast form was any less of a magus, since it was no longer truly human.
Magi who become Faeries or otherwise change realm no longer have the Gift.
The Sahirs rule that anything that can be summoned is not a Sahir. This is probably irrelevant among magi. (It matters to Sahirs since they do everything via spirits; a clear distinction between spirit and sahir is important. Sahirs also don't care about Realms very much; magi do.)
Also probably irrelevant: At the Tribunal weekend held in Silicon Valley some years back, I recall a preference not to rule upon the issue, which was a storyline brought on by a player whose Faerie magus (all 3 Becomings) insisted, with an emerging near-total but informal consensus that a faerie could not be a magus but that the issue should be tabled until after the character was killed; meanwhile, another magus reputed to have reached Final Twilight decades before was accorded full respect, though this might have changed had he claimed the rights of senior magus present.
In AM2 core rules, Grimgroth's familiar became a maga. But that was 3 editions ago.
I suspect that if the creature had the Gift and its Hermetic Arts were opened, most Tribunals would accept it... but the character might have a story flaw to represent those magi who distrust or revile him. Outsider, Enemies, Judged Unfairly, some bad reputation...
Have fun.
Anyway,
Ken