Jainicorn

Jainicorn is the het ship between Jake the Dog and Lady Rainicorn from the Adventure Time fandom.

Canon
A long time ago, at least a thousand years before the Mushroom War, the Rainicorns and the dog people were at war. It is unknown how long it lasted, but the Rainicorns left to live in the Crystal Dimension. Before the events of the series, Jake met Lady Rainicorn. They started dating. The details are not known.

In the first episode, Finn convince the two lovers to go into a closet so he can keep the zombie invasion secret. In My Two Favorite People, Jake tries to get his brother (Finn) and his girlfriend (Lady Rainicorn) to get to know each other. But that works too well, and Jake gets left behind by both of them.

The two did a "tier 15" next to the Music Hole. And some times later, Lady Rainicorn gives life to the first generation of Pups.

Fanon
With Jainicorn being canon since episode 1, it's rather popular. It doesn't seem to have a lot of content created for it. (Surprisingly, no one talks about Lady Rainicorn after the main series.) On some rare occasions, fans ship the couple and Prismo together.

Fandom
FAN FICTION
 * Jake the Dog/Lady Rainicorn tag on AO3