"They're here, they're horned, and they need your help," according to a RIFT' website update. Trion's latest in-game event features unicorn refugees from the Plane of Life, who of course require plenty of assistance from RIFT's ascended players.
Trion also wants you to know that you can acquire onyx unicorn mounts as rare drops from the Hooves and Horns zone event in Moonshade Highlands or by completing the Rainbow in the Dark achievement. Alternatively, you can buy them in the game's cash shop. Opal unicorn mounts can be similarly purchased or acquired through daily quests in Sanctum and Meridian.