Those sound like keratoses. Many hypotheses have been advanced for why they can rarely happen but I do not think that definite causes or treatments yet exist though you can search on them to see and certainly ask your vet. In about 29 years w ferrets we have had only one get them. Back then adding zinc was tried and it was useless for our ferret. In Hjalmer's case perhaps developing lymphoma might have created a vulnerability. If they become large enough they can cause some problems walking. [Posted in FML 6906]