Atlas (horse)
Atlas was a horse that Brad Boimler possibly hallucinated when he was "clinically dead", though it is possible Boimler saw him in some kind of vision.
Atlas was with Hikaru Sulu at James T. Kirk's uncle's old ranch in Idaho, and Sulu told Boimler that Kirk was gone traveling through time or something like that, which was apparently why he was there watching over his ranch.
After Boimler talked to him about the meaning of life, and Sulu told him that he had lost a lot of friends both tragically and heroically. He then told him it was best not to dwell on the fact that a person you loved died for no reason, as it would just get in the way of being the best you could be.
After asking Sulu a bunch of questions about what it was like back in his day, when he had served on the original Enterprise, he told him "The horsey [was] going to bite [him]". Just then, the horse bit Boimler on the neck, and he found himself waking up in the sickbay of the USS Cerritos. (LD: "Crisis Point 2: Paradoxus")