I just read Calebais last chapter with my spouse. Her Merinita maga became friend with Pitt Patrick, a Rhool explorer, and she wishes to make him a long-time companion. Sadly rhools only have 20 years lifespan which is short regarding hermetic standards. Her and I wondered if a rhool may become a familiar.
On the one side calebais book does state that animula cannot be a magus' familiar. On the other side rhools aren't categorized as animula ; someone may argue that the covenant magical aura may have altered the primitive (animulae) rhools, changing them into magical beasts (they are indeed). Rhools haven't animula Vice in their sheet.
If not, would you bat an eye if a maga invent a longevity ritual for a rhool ?
The rules about Longevity Rituals talk about a magical anchor that sustains vital forces, often affecting body tissues. As animals have vital forces (they wouldn't die otherwise) and have body tissues, I don't see why not. Only if the ritual involved a Mentem part I would have troubles, but as written it seems to fall fully under the form of Corpus, so it could be translated to Animal.
Humans have an average lifespan of 50 years, so given the 20 years average lifespan and according to beast aging rules in Grogs, p. 138, numbers should be adjusted by a 2.5 factor. So they should start making aging rolls at age 14, add one to the aging roll every 4 years and make 2.5 aging rolls per year (I don't know how to manage that half-roll: 2 rolls on even years and 3 on odd ones, or something like that, maybe). Obviously the ritual should use the CrAn lab total instead of CrCo, your wife should have a CrAn lab total over 30 to work the ritual.
The benefits are -1 to the aging roll for every 5 points of lab total for magus (or gifted people in general, probably) or anyone with a supernatural ability and -1 for every 10 points for non-magical people, and I don't know where these little furry beasts fit. They are magical, as they have magic might, so I guess they would get the -1 per 5 points full benefit (...or maybe the factor should also be considered here, and modify the roll by 1 per 12.5 points of lab total).
And the final issue would be the vis costs... for humans the rulebook says it's 1 pawn per five years of age, so I guess that it could be reasonable to increase it by the 2.5 factor and made it cost 1 pawn per 2 years of age, and given the high ammount of aging rolls, I guess she's going to harvest quite a lot of vis just for this.
I think a Hrool must be an animula, as the final chapter of 5th edition Calebais says immediately under Awakening Animals "To duplicate Ierimyra's work" suggesting that the rules that follow are how Ierimyra's creations were made. As Hrools are from a Hermetic Breakthrough they don't necessarily have to follow the usual rules, so I think the part about Awakened Animals not being able to be familiars should apply to Hrools as well.
There was recently a thread about familiars and if they can't have a soul or must have a soul. In the Rules As Written, anything other than a regular animal or magical animal can only be bound by someone with a special virtue. Are Hrools' unusual nature and magical creation enough to make them non-standard animals, and therefore not bindable by regular magi? I think so.
As always, what works for your saga best should always apply.