From Wiktionary, the free dictionary
From mer- + friend.
merfriend (plural merfriends)
- (fantasy) A friend who is a merperson.
2012, Debbie Dadey, Trouble at Trident Academy, Aladdin, →ISBN, page 76:“I told you that you are the best merfriend ever. Let's go sit with her and make her feel at home.”
2014, Despina Karfis, The Magical Pool, Trafford Publishing, →ISBN, page 7:Helen asked Eleni, “Would you like to join me and my merfriends on an adventure?”
2015, Jillian Verbicky, The Tail of an Angel[1], Xlibris, →ISBN:“I love singing, and I'm pretty good says my, umm, merfriend, if that's even a word.”