It's a Dog Guardian from Nuclear Throne. http://nuclear-throne.wikia.com/wiki/Dog_Guardian They are really quite annoying if you can't knockback them quickly enough.
u see this rad pupper walking around your neighborhood and u think ' man thats a cool ass dog' it tries to hug you, killing you instantly YOU DID NOT REACH THE NUCLEAR THRONE
http://nuclear-throne.wikia.com/wiki/Dog_Guardian
They are really quite annoying if you can't knockback them quickly enough.