The Ceffyl were one of the non-human races on Tír na n-Óg which were created by Goibhnie. They resembled unicorns, their horns being a direct extension of their brains; if their horns were cut off, they became ordinary horses. Ceffyl were telepathic and could form a bond with a human once in their lifetimes, allowing them to communicate with each other. They were also precognitive, as they were not entirely bound to the present.
When Tir Na N-Óg's sun vanished, some Ceffyl attempted to go through the gateway which connected their world to Earth. Stuart Taylor found one of their horns, which compelled him to go through the gateway on a quest to see Goibhnie. Another herd of Ceffyl was led through the gateway by Ace, but the human soldiers guarding the gateway caught up with them and cut off their horns. They were stranded on Earth when the gateway was destroyed. (PROSE: Cat's Cradle: Witch Mark)