I have a couple Beaver that are going to be brain tanned. Now disclaimer only one I've done so far was with the orange bottle stuff but it is my understanding to not get anything on hair side. Hopefully someone with more knowledge will join in with adult advice. One thing I've learned with beaver is it is thick along the back which will need thinned however you prefer. As far as eggs the only thing I have done is a mole with mixture of mole brain and egg. Turned out good actually. Have no info on Otter but brains works on most everything I've messed with. Again, Kind of miss Bow Ed on hide things.
Piddler