One of the defensive behaviors wild hogs do is spew a thick dark blood from their mouth. I have better pics if you would wish to see that close up.
Whats interesting is, Its shuts off as soon as the threat is past. Like they have the ability to start and stop it. And it can be a lot of blood.
Not every encounter results in mouth bleeding, but its common.
When they do it, they commonly mouth their own body and spread it all over. Once in captivity, I have not seen it occur.