HTF OCs meet Ponies is a fanfic by RoboStarthebomb on deviantart.
The story has fanon characters in it (although Nutty, Pop, and Cub appear). It invovles Twilight making a machine involving the ability to travel to distant places in Equestria but the CMC went inside and caused the machine to malfunction, causing them to teleport to Tree Town.
Later the Mane 6 and Spike find out about this and enter the machine hoping to find the CMC and rescue them. Along the way they encounter certain fanon and canon characters.
The story is not finished yet.