The Chimera is a mythological creature (as indicated by Rose) that's known for its cruelty and monstrous body. Only one appears in the entire series. It was trapped deep into Professor Rotwood's subconscious and accidentally set loose by Jake Long.
In the series, the Chimera is a three-headed humanoid creature that is able to walk on its hind legs. It has the body and head of a lion, the head of a goat, and the head of a cobra. It also has the tail of snake. Its entire body is a mix of red and purple colors.
A Chimera in general is very powerful. It has superhuman strength and, although this only applies in the Dream Realm, can cause physical harm to humans. Whenever it appears into someone's dreams, it will cause horrifying nightmares that will make that person grumpy and/or sleepy the next morning.
The Chimera is incapable of speech and often acts from its instincts. It is extremely dangerous when on the loose as it will invade all the dreams it can in one night to cause as much havoc as possible.