Not a fibroma (a dark-colored, wart-like growth that eventually falls off). This is a seroma, which is a bag or pocket of skin filled with sera or other body fluids. Initial cause is an injury, usually penetrating (frequently antler tines, but not exclusively so). Not anything contagious. This location on the body is the most commonly-seen area. Normally it ruptures and drains. Although the body condition may be due to a completely unrelated cause, it would appear that this one has become infectious, likely causing septicemia.
The recreational value of game is inverse to the artificiality of its origin - Aldo Leopold