Rhylo was the pet dog of Keyla, a mysterious woman that Jonathan Archer met on Risa who turned out to be a disguised Tandaran. Rhylo was a female and her body was primarily devoid of fur, with the exception of her head, feet, and tail.
Keyla claimed to have adopted Rhylo when she was a puppy. Prior to their visit to Risa, Rhylo had been "cooped up for weeks." This was the excuse Keyla gave for Rhylo's actions when the dog apparently ran off and somehow appeared on the deck of Archer's villa, where she started a growling match with Archer's dog, Porthos. (ENT: "Two Days and Two Nights")