Beast infos, slightly spoilerish
It's a lion in a bone-and-metal shell. It's never explicitly described as such, only that Jean-François brought a "strange animal" back from Africa and kept the strongest of its younglings - but as a lion attacked Jean-François, it's rather sure that he brought one of them back home out of strange fascination.
The Beast has kind of four jaws - its normal ones with implanted steel teeth and a second set of jaws made from a big skull around its head that is operated by a complicated string construction, with bigger fangs than a normal lion could ever have, the biggest made out of metal. And it has metal spikes attached to his back. Don't ask me how it even moves so fast with all that weight (which is about 500 pounds!) , but it does. It also jumps with ease and can climb not-to-steep walls. It's never said explicitly in canon, but implied, that Jean-François trained it to wear this armour since it was young.
The beast is very fast, so fast that I had a hard time finding any non-blurry screencaps. It's also very strong, in one scene it breaks through the floor of a wooden house and rips apart the supporting timber with only so much of a shrug. It breaks through a wooden cage when Fronsac and Mani try to catch it. It is hit by a spiked tree log on chains and only gets a bit dizzy afterwards. In other words, the beast is tough and also ignores pain rather well - it was trained with "patience and cruelty" as it is said.
The beast is not invulnerable, however. The part of the armour that is bones, i. e. mostly the head, can be crushed with a well aimed blow - Mani hits it with his tomahawk and draws blood, Fronsac wounds it with a shot. It's just really hard to hit relevant spots, as they are difficult to aim at. Against people with supernatural abilities, the Beast has not really a chance. If you're a mere human, you have to be armed and most importantly, very fast.