Mutates are infested terrans subjected to anti-zerg nanite treatments which restored or preserved their intelligence. However, their appearance and sanity left something to be desired - the nanites were all that kept them from reverting to their zerg instincts. These mutates are armed with sharp claws and have a spine-hurling organ in the chest.
A terran force defeated a small zerg brood and captured it during the Brood War. Mixing zerg and terran genes, bioengineers forged a small number of mutates to use against the zerg. They ended up in the service of the Alliance.