Unicornia
From My Little Wiki
Unicornia is the home of unicorns in the third generation of My Little Pony. The location features a relatively big village with houses separated by a river with bridges and a rainbow waterfall. On top of a large hill is Crystal Rainbow Castle located.
The Runaway Rainbow
The Run Away Rainbows story revolves mostly around Unicornia and how Rarity faces a challenge in getting back in time for the Princess Rainbow ceremony.