While Martin Brundle is a child growing up at Bartok Industries, he befriends a cute golden retriever used as a lab animal there. He often sneaks lamb chops to him. One day, though, he learns that the dog is going to be used in some kind of experiment, and so wanders into Bay 17 where he discovers what we know to be Seth Brundle's two remaining telepods from the first movie. Hiding in the observation booth, Martin watches as the golden retriever is placed inside the first pod. Completely unaware of what is about to transpire, he happily waves to it, only to be shocked as the dog vanishes, having been teleported from the first pod to the second.
A lab technician goes and opens the second pod, from which an ominous fog rolls. Initially it seems as if the dog is fine, as the technician announces that it's alive and moving. However, the telepods were damaged and the teleportation was only partially successful. Although the golden retriever is alive, it has been reassembled wrong. Hideously deformed and vicious, it lashes out and bites off several of the technician's fingers before the scientist's can restrain it.
Martin's horrified reaction grabs the attention of Bartok and Dr. Jainway standing nearby, and, still pretending to be the loving father figure, Bartok comforts Martin and assures him the animal will be euthanized. Years later, though, the now adult Martin is at some kind of office party when he overhears a few off-duty researchers discussing some kind of experimental animal they keep locked away in secret.
They call it "Timex," because, like the watch brand, it "takes a lickin' and keeps on tickin'."
Being a naturally curious fellow, Martin decides to sneak on over to where they work (using his high-level clearance) and see just what it is. What he discovers breaks his heart and changes his opinion of Anton Bartok irrevocably. It is the golden retriever. Still deformed and monstrous, kept in a deep, dank concrete pit with straw at the bottom. As Martin watches, the chained-up creature pitifully crawls over to its food dish which is filled with a disgusting-looking whitish slop.
It is apparently in great pain, as evidenced by the howl it issues forth at one point. Martin breaks down crying from witnessing his former friend's pitiful fate. Bartok lied to him. Sometime later, when no one else is around, a determined-looking Martin goes down into the dog-mutant's pit alone, armed with a cloth and a bottle of chloroform. These he sets aside. The dog-thing initially growls at him ferociously, and seems about to attack him.
However, as he tries to soothe it, it remarkably recognizes him, and begins to wag what can charitably be called a tail. Becoming teary-eyed once more, Martin sits down alongside the dog and pets its malformed head as it whines. After spending a few final moments with the dog, Martin makes his move. He douses the cloth he brought in way too much chloroform and places it over the former golden retriever's nose, euthanizing it himself and ending its suffering.