Here we have a pair of graveyard-stuffing six-drops for Limited. The Carriage makes sure there's always room for one more card to play from your graveyard, while the big black (Demon) Dog will punish your opponent for anything from dying decayed Zombies to mill.
Probably for the same reason they don't tack on the horse type with knights riding horses. Since it's the "carriage" they probably consider it whoever is steering (piloting? Iunno) though I get where you are coming from the way the art is focused.