A Corusdeer is, as the name implies, a monster that resembles a deer.
It looks like a deer, except for being twice as big as one. Furthermore, it has two antlers on it's head, which are orange/brown colored.
A Corusdeer was shown to have battled against Toren, while he was trying to level up. Toren managed to win, though only because Toren could constantly reassemble himself.
it can superheat its horns sufficiently to vaporize Torren's ribs.
Even Gold-rank adventurers don’t like taking on a herd of Corusdeer. 
- Liscor's adventurer guild pays four silver for each antler, while according to Octavia: "one horn can go for as much as twelve silver pieces or double that if there’s a shortage".
- Corusdeer love killing Snow Golems, as they are their natural enemies.