The Black Beast of Argh appeared in the 1975 movie called Monty Python and the Holy Grail. The monster obviously behaves much like other terrifying man-eating monsters such as Dinocroc or Supergator, where all of them are relentless in the pursuit for their prey, particurlary humans. It's unknown whether he also rivaled Rabbit of Caerbannog or not considering the fact that they are man-eating beasts.
The beast's appearance is obviously reptilian albeit only having two legs, with numerous eyes and teeth, brown hair on his head, and a pair of horns. Though the depiction supposedly showed him as a terrifying black beast, he's instead depicted as a hilariously ugly green monster.