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]