I apologise if this has been handled recently ('search' didn't help me), or if the answer is just obvious and I'm being thick.
If a character with the Shapeshifter virtue changes into their animal form, do they have to penetrate in order to physically attack a creature with magic resistance?
Skinchanger is the same as using a spell, so you do need to Penetrate (same page ref as above). Lycanthrope works the same as Shapeshifter, IIRC. The key point is that Heartbeast and Shapeshifter (and Lycanthrope) use magic to effect the change, but once that has happened there is no ongoing magical effect; the target is not a magical medium. Skinchanger and spells like Shape of the Woodland Prowler have magic actively maintaining the new form, so the target is a magical medium and needs to Penetrate.