A fibroma is a benign tumor of the connective tissue.
Fibroma usually grow quickly and may reach the size of a hazelnut or even bigger. They often erode at their surface and break open. Upon handling, a fibroma feels firm or even hard. Trunk and limbs are common sites for this tumor to develop.
The underlying cause of a fibroma remains mostly unknown.
If the nature of a suspicous lump (benign or malign) is unclear, a fibroma would often be removed for safety reasons.
If you have noticed a suspicous lump on your dogs skin, consult a veterinarian.