In Greek mythology, the Chimera was a monstrous fire-breathing hybrid creature composed of the parts of more than one animal. It is usually depicted as a lion, with the head of a goat arising from its back, and a tail that might end with a snake’s head, though in D&D is more likely represented as a three-headed beast sharing traits from a lion, a dragon and a goat. The term Chimera has come to describe any mythical or fictional animal with parts taken from various animals, as you can see using Merriam-Webster dictionary.
A chimera has the hindquarters of a goat, the forequarters of a lion, and a set of dragon wings. The beast has three heads: a horned goat, a lioness, and a dragon. The dragon head can be black, blue, green, red, or white, and has a breath weapon appropriate to the color. They rarely mate; their creation is the result of «a dark mystery better left unexplained», according to the 2E Monstrous Manual.
Society and ecology
The chimera, being a hybrid, combines the preferences of the lion, the goat, and the dragon in its habitat, society and ecology. The dragonish part of its nature gives the chimera a distinct preference for caves as lairs. The dragon and lion parts seem to war with one another, for some chimerae are dragon-like in their preference for solitude, while others live in small prides.
The monster is an omnivore. The goat head will browse on the toughest plants and shrubs and will derive nutrition from the most barren vegetation while the lion and dragon heads can only be satisfied with flesh. The chimera hunts once every three to four days, using its strength and limited intelligence to gain an advantage over those it preys upon. Having a voracious appetite, it sometimes roams over territories as large as twenty square miles. Being chaotic evil in nature, the chimera enjoys preying upon men, elves, dwarves, and halflings. It will even gladly attack other monsters in its search for food. Anyone entering its territory becomes prey, and will be treated accordingly.
The chimera cannot resist attacking groups of travelers or monsters for another reason: its dragon nature craves the treasure that its prey might be carrying. Although it has no earthly use for it, the chimera will gather the coins of its fallen foe into a heap and roost on it like a dragon. Its hoard is nothing like that of a true dragon, however, and consists mainly of copper and silver coins, with perhaps some jewelry and a few magical items.
The chimera fills the role of both omnivore and a top predator in its ecosystem. It is very adaptable. During times when its prey is scarce or non-existent, the chimera can make do with a vegetarian diet.
In the Eberron campaign setting, the chimera is the heraldic beast of House Deneith. As such, in my campaigns, the highest-ranking members of the Defenders Guild have access to a magic ring that allows them to summon a chimera to fight along with the against their enemies.
In your campaign
Using the Chimera as an encounter is not difficult at all, being an aggresive predator frequently looking for prey. But you might want to give a twist for your players when they hear rumors of a chimera in a town and find out the word is used in its more-modern meaning: a twisted wizard combining different creatures in oorder to create the ultimate hunter to defeat his nemesis. Even more dramatically, you can use a variation of the storyline featured in the Fullmetal Alchemist: Brotherhood episode «An Alchemist’s Anguish», in which the antagonist combines his daughter and pet into a single creature.