From mer- + princess.
merprincess (plural merprincesses)
- (fantasy) A mermaid who is a princess.
1997, Carleton Papers in Applied Language Studies, volumes 14-19, page 109:Ariel is a merprincess who finds life in the sea under the protective eye of her father to be a type of imprisonment.
2003, Gretel Killeen, My Sister's a Sea Slug[1], Random House Children's Publishing UK, published 2012, →ISBN:We have the merprince and merprincess and tonight we will finish defeating the soldiers at the mercastle.
2013, Cheri Jones, The Adventures of a Mermaid Princess, Trafford Publishing, →ISBN, page 26:“As long as that merprincess is out there, the more that city will crumble and die. […]