I'm sorry I don't have any experience with this, and don't know what it could be other than a cyst, but hopefully someone with experience will come along soon, to offer some advice. I just wanted to say I hope Sherman is okay, and it's nothing serious. These guys give us so much love and joy, but they give us such worries too. Sending love, hugs and prayers to you and Sherman. Feel better buddy. Maybe you could call the vet and ask what it could be?