The Mercora were a peaceful race that came to Earth and colonized it during its prehistory.
The Mercora were huge, asymmetrical multi-colored crabs with one big three-way pincher on one side of their body, and three human-like arms, with long, tampered, delicate fingers on the other side. They had seven legs in total, three on one side, four larger legs on the other side, as well as multiple eyes, which were each hidden beneath tiny trap doors in their shell, and they communicated by Thought-speak. Like most crustaceans they are able to regenerate their legs.
Around 65,000,000 B.C. the Mercora home planet was destroyed when its sun was sucked into a black hole. Majority of the species relocated to prehistoric Earth and created a colony. They were constantly at war with the Nesk over Earth.
The Mercora encountered the time stranded Animorphs and provided them shelter. After conversing with Ax they surmised that the only way to return them back to their time was to acquire a nuclear device from the Nesk. The Mercora were unwilling to commit an attack on the Nesk encampment due to the limited military resources, forcing the Animorphs to infiltrate the base as dinosaurs. After their time displaced Animorphs were discovered, the Mercora came to their aid, losing two of their ships in the process.
In an act of spite the Nesk changed a meteor's trajectory to collide with the Mercora colony. Seeing that their doom was approaching the Mercora appealed to the Animorphs to use the nuclear device and install it on their last remaining ship to destroy the object. The Animorphs knowing that they could not change the future of Earth, turned the nuke into a dude and allowed the destruction of the Mercora.
The Mecora were a spacefaring species with technology specialized in using forcefields. They were herbivores and grew a variety of vegetables including broccoli, a crop they brought from their birth world. They were a peaceful species and abhor violence seeing it as a sin. Should they do commit an intentional death, they undergo a self-mutilation ritual in which they cut off one their legs as a symbol of the pain they inflicted.
- Megamorphs#2: In the Time of Dinosaurs.