Cyborg is a playable species. They are availible for all classes for free-to-play accounts.
Cyborgs are individuals who have undergone surgery to either replace or enhance parts of their natural bodies. Augmentations can range anywhere from simple subdermal implants, to full body prosthesis. As of the game's initial pre-release access, only former humans are playable cyborgs, though Rattataki varients can be seen during the Imperial version of the Korriban Incursion tactical flashpoint.
PC Gamer UK magazine stated in their May 2011 issue that Cyborg would be one of the playable species.
One image that appears to be a human wearing a face implant is referred to as a Cyborg, and according to the accompanying chart, Cyborgs could be either a Trooper, Smuggler, Imperial Agent, Sith Warrior, Sith Inquisitor or Bounty Hunter class. However, this was later discounted by Daniel Erikson, who announced that some species were being used by various classes for test purposes, and that Cyborg won't definitely be a playable species.
It was confirmed 6 months later that Cyborgs are actually a playable species by James Ohlen.