Cos is an alien that appeared in the DS9 episode "Rivals". He was portrayed by Albert Henderson.
Cos appeared as a prisoner in DS9's security office, sharing a cell with El-Aurian con artist Martus Mazur. He appeared to be in poor health, which he blamed on a small spherical device he was carrying. The device appeared to be little more than a toy, but it was capable of altering the laws of probability. Cos stated that he had used the sphere to gain great wealth, but the device also caused him to later plunge into financial ruin. He had held on to the device for years after and played it one more time in his cell. He apparently won the game and he suddenly died with a contented expression on his face.
After Cos' death, Mazur took his gambling device and used it to try and con the residents of DS9.