Rhinormous evolves from Rhinoggin when fused with an Unbeatable Soul.
Rhinormous appears as a stout humanoid Hercules beetle with a brown shell, clad in lightish red samurai armor with golden accents, with the horn poking through his helmet, and white hair peeking out from under it. His shoulder pads and armguards are secured by blue straps, and a belt of the same color is tied around his waist.
Fuse Rhinoggin with an Unbeatable Soul, which can be obtained post-game by trading two rare Anglerfish and three rare Rhino Beetles at Jungle Hunter. Afterwards, more Unbeatable Souls can be found in Terror Time chests.
Also, Rhinormous rarely appears on trees by The Fish Place at night (Uptown Springdale).
His Japanese name, "Kabuto Musō", means peerless kabuto (mushi) or peerless helmet. His English name, "Rhinormous", is a portmanteau of rhino (beetle) and enormous or, colloquially, ginormous.
Like its pre-evolution, Rhinormous is based on a rhinoceros beetle, but more specifically his girth and shape of his horn remind of a Hercules beetle, the largest species in its genus, as well as the longest extant beetle species.