Alicorns

Alicorns are pony characters in the show who have both a unicorn horn and Pegasus wings, who are all styled as royalty.

The Abadeers
Marceline and her father and sister are introduced as Alicorns. Mathison Chicetawn stated that Hunson Abadeer was originally going to be a unicorn, but was changed due to a reccent picture of him and Marceline. Marsha and Marceline and even Hunson embodies the traits of all three pony kinds: Earth, Pegasus, and unicorn. According to other pictures of Marceline pony-i-fied, she was supposted to be a pegasus, but was changed at Mathison's request.

The Abadoes
Like Marshall and his mom and brother, they are alicorns as well. They too embody the three pony kinds. Marshall was once a pegasus, too. But he was changed since Marceline was.

Other Alicorns
Princess Bubblegum and her sisters are alicorns, even though they can't fly nor have magical abilities, it's just that they are royalty. The same thing is for Gumball and his brothers.

Princess Bubblegum and Gumball were both once unicorns but were changed to alicorns at Mathison's request