Ok, brace yourself, it’s the origin of Comet, the Super-Horse in Action 293 (Oct. 62), by Leo Dorfman and Jim Mooney.
Linda is still vacationing at the ranch, and while out for a ride on Comet, he telepathically relates his story to her. Long ago, in ancient Greece, he was a centaur, Biron. Due to the manipulations of an evil wizard, Circe wound up turning him into a horse.
Feeling guilty about this, Circe devised a magic potion to give him various super-powers, including telepathy, and immortality (which explains why he has been around since then, and will still be around in the time of the Legion of Super-Heroes).
Comet saw Supergirl, while floating aimlessly through space, after being sucked into the void by a comet, and made his way back to Earth. His story done (for now, more will come in a later issue), they fight off an alien invasion together. Comet had known about this, and telepathically sent her dreams about himself, so that they would meet.
Just as the reader’s brain is about to melt from all of this, Linda discovers that Comet has been sold to a Hollywood producer. Oh, No!