One thing I'm a bit unclear about after reading RoP: Magic is the lifespan of Magical Animals. There seem to be some contradictions in the Ars Magica products, especially the Broken Covenant of Calabeis. The Hrools, being transformed from ferrets into intelligent creatures, are described as still having the life span of ferrets (8-10 years on average). But aren't creatures with Magic Might immortal, able to live forever if they have a Magic Aura strong enough to support their Might?
Another question. A Bjornaer Magus entering Final Twilight as a Great Beast, could he be played as a Magic Animal or Magic Spirit character? Any ideas? What would a good Might Score of a Great Beast be? If they even have Might? Any guidelines and ideas on how to create a Great Beast using the Magic Character rules?
I'm not completely sure about the hrool (don't have BCoC).
However, I have heard from some here on the board that BCoC was essentially a transitional adventure that didn't perfectly conform to the 5th edition rules. In particular, the Hrool (IMO) seem more closely aligned to the realm of Faerie than to Magic.
The rules don't specifically state that creatures with Magical Might are Immortal but rather that they don't age. If it's important to your saga, you might reconcile this with the Hrool's limited life-span by imagining the unchanging quality of magical beings as being similar to the Unaging virtue. Indeed, the fact that magical beings are vulnerable to the Age-Quickly flaw seems to suggest this being the case.
As per the Bjornaer. I've always assumed them to become "Beasts of Virtue". The rules in RoP: M use the "Magic Animal" virtue to refer to anything with an animal-like shape (dragons for example) so that's what I'd do. I suppose if you had a plant Heartbeast then "Magic Thing" would be appropriate, But I can't see "Magic Spirit" fitting, unless the character in question used one of the Mysteries to generate an incorporeal "Greater Heartbeast" of one sort or another.
If you take the Age Quickly flaw, you'll age normally. See page 47, third column.
Yes. Most likely a Magic Animal, although some Inner Heartbeasts (such as the shadow wolf example) might be equivalent to spirits.
HoH:MC gives them a Magic Might of 5x Heartbeast Ability score.
Well, the specific power list is highly dependent on the former magus. But with a number of Magic Qualities equal to the beast's Might Score, you've got a lot to work with.
As for what should be inherited to create a basic "Great Beast Template", I don't know.