On Twitter, Kojima has stated that Venom Snake, aside from his horn, also has a tail, or at least, something that reminds of a tail.
There’s a horn on V Snake but in fact he also has a tail. Instead of physical tail, the belt looks like the tail.
On this screenshot the belt can be seen hanging from Snake’s back. It’s visible on other screenshots as well.
These physical attributes are meant to give Snake a demon-like appearance.
Source: Hideo Kojima Twitter