If normal animals can hurt lycanthropes, I think game theory needs to match up with it.
If the animal is able to damage the beat via the spirit of the wood, we have presidence (since the same spirit powers Druids, Animist, Rangers, etc). How ever, as with the use of all power, danger is involved, and this time the danger is very personal and known at a deep instinctive level.
I prefer simplicity. So, all animal attacks against lychanthropes will be 1/2 hits and Crit II. A crits will resolve at -40, B cits at -20.
The animal must make an rr against corruption. The attack is the level of the were, target resist at own level. Target recieves no bonuses to roll but suffers a -15 to rr per level of critical severity delivered ( A is -15, B is -30, etc). Number of points are doubled if were is bleeding (unless GM declares blood fails to enter mouth/beak/etc) Amount roll is failed by is ammount of corruption points recieved.
If a PC uses a spell to summon or enhance the animal, then it suffers no penalties to damage and will recieve a bonus to all RR's equal to Tpre x5. (So, a Summons III wpuld provide +15 to all rr's for the summoned animal, of course).
Ugh...ended a sentence with a prepositional phrase