A cyberdog, more specifically "cyberhound Mk II personal security model". Dr. Henry brought it from the Enclave Oil Rig and settled down in the NCR. The robodog has stayed there with Henry, not fully functional.
Dr. Henry, afraid that too much information about the Enclave would be obtained from K-9, attempted to destroy the cyborg, but was stopped by Robodog and Dorothy, who suspected what Dr. Henry was planning. The NCR government used the attack as an excuse to confiscate K-9 and cyberdog in order to learn more information about the Enclave as well as what makes the two of them "tick." At last report (and over Dorothy's objections), K-9 and Robodog were disassembled and analyzed. Structural damage during the disassembling is reported to have killed them both.