A person who has amazing sexual prowess, or a person that is really awesome.
It can also mean that a person is a complete douche depending on how you phrase the word.
"That guy always has a shit load of girls...what a cunt stallion!!!"
"Dude this guy just saved my life!!! He's such a cunt stallion!!!"
"What the fuck? Who is the cunt stallion that shit on the coats?!"