I've tried looking through all the results for "creo + heartbeast" in the forum to check this hasn't been answered elsewhere, so here goes.
We know that a magus can create Creo Corpus rituals to boost their physical characteristics, and Creo Mentem to boost their mental ones. Creo Animal allows you to do the same to animals, with a limit of +5 past the normal average for their race (so a bored magus could boost his familiar something silly if he had the vis, and I have plans for a Merinita Animal specialist boosting an entire bloodline of domesticated animals). What happens when a Bjornaer does these rituals - do his Corpus rituals still boost him in animal form? Do his Mentem? If he casts Creo Animal ones to boost his heartbeast, do they carry over to human form? For a shapechanger, we could clearly say no - but a Bjornaer has both human and animal as Essential Nature, which is why I'm getting confused.
According to the guidelines from HoH: Mystery Cults page 23, the characteristics upgrades from Creo Corpus/Mentem will translate into the Hearbeast. As far as I can see, there'd be no problems doing it with Creo Animal as well.
Also, the Creo Animal part would not carry over to the human form, as the Hearbeast does not influence the human statistics normally.
If you have an already strong animal form, like say a bear (Str +6), and you Creo Animal it for a boost of up to an additional +5, you can get some impressive results.
I'd been wondering about the same thing not too long ago. It seems the only real rule we have to use is that effects on the animal shape remain in human shape and vice versa. This is where I figured that leads:
Non-heartbeast shapeshifter: The animal is just not a fundamental nature and so it doesn't really have its own inherent scores to raise. CrCo always works and CrAn works if in animal form. These increase the human scores up to the limit of the ritual. Animal scores can be recalculated from the new human scores as normal.
Hearbeast shapeshifter: The human and animal are their own beings, but spells affect both if they work. CrCo works in human form; CrAn works in heartbeast form. In either case the score of both the heartbeast (before modification) and the human form are raised a point, up to the limit of the spell. (Note that the limit may be reached in one case and not the other.) Animal scores can be recalculated from the new human and new Heartbeast scores as normal.
There is another issue, too: when to apply Qualities versus such an increase. For consistency I always apply Qualities after animal scores are calculated from base values and Virtues/Flaws. This helps prevent min/maxing issues. So, for example, if an animal's base Communication is -2 and it has Vocal, its Communication is 0. Then what I do is allow increased from the -2 point up to a maximum of +3. I then apply Vocal afterward. So no increase is 0, and increase by 1 is 0, an increase by 2 is +1, and increase of 3 is +2, and an increase of 4 is +3.
Thanks for the ideas folks, some solid opinions there.
After re-examining the heartbeast section of HoH:MC, I looked at the cost of Inner Heartbeast rituals and discovered that raising characteristics is a difficulty 18 (so 6 pawns of vis, 1 warping point, 1 season, you must make initiation total of 18 including the +7 bonus from the script) so for you inner heartbeast it's probably cheaper to do the first improvement for each characteristic this way, and switch to the CrAn rituals later on when the difficulty levels get silly.
Rival Magic has rules for Muspelli Jotun-form, which is basically Heartbeast on Norse steroids (turn into a giant! Ignore General flaws like being blind or lame while in this form! Be huge and monstrous, but beware the fact you risk fatigue and exhaustion every hour!). There, they can take the venomous quality more than once and slowly improve the quality of their venom. Mystery Cults implies you take venomous once, and the SG applies what they feel is the right venom for that particular poisonous species. If you can take venomous multiple times, it would be tempting to take adder Heartbeast (from the book of mundane beasts), improve your venom to awesome levels of toxicity, and improve the starting quickness of +3 to levels so high any human is unlikely to react in time.
Thinking of which, as a heartbeast is truly a member of that species, an Inner heartbeast snake's venom should count as completely natural and so bypass Parma. Does your Animal form bonus add to the Stamina roll against poison, or just against their damage?
Hmmm...the toughest animal species have both high Sta and Toughness, so you get really good soak and a bonus to casting totals - now I see why bears, wolves and lions are so popular. On the other hand, the tiny ones have ludicrous quickness and dexterity while being so physically puny one good hit and they're dead - great if you want to attack with an aimed spell before they can respond.