Did you miss out on RIFT's Unicornalia event last May? Well, it's back, according to a post on the fantasy MMO's website. Refugee unicorns from the Plane of Life require your assistance, and you'll need to "find the PRI representatives in your capital city to discover how you can help."
Onyx Unicorn mounts are obtainable via rare drops from the Hooves and Horns event in Moonshade Highlands, as a reward for completing the Rainbow in the Dark achievement, or from the game's cash shop. Opal Unicorn mounts can be had from daily quests or the store.