This can be a potential sequel plot where he becomes too troublesome for Maisie to keep around and they have to find him a new home.
The tail fluke resembles Red but is more ornately adorned and patterned, similar to early Concept Art of Red. Perhaps this is a male Red Bluster, with Red's smooth and plain-looking appearance due to her being a female.
The sequel will be set years after the events of The Sea Beast where hunters like Sarah Sharpe have adapted to change and now into commercial shipping; using the monsters they once hunted to move heavy freight around the world, and others, as Living Ships to ferry people.
Only several centuries have passed between events of HTTYD3 and this film.
The HTTYD movies never really explore beyond what is presumably the northern Atlantic, while the general climate of Sea Beast is clearly tropical, or at least Mediterranean. And it's not like there aren't fictional non-dragon creatures in the HTTYD franchise (like the gigantic eels in Race to the Edge). Simply put, Sea Beast is set in a warmer part of the ocean further south, several hundred years after after all the dragons left for the Hidden World. Later generations of people have basically forgotten that dragons ever existed, with focus shifting toward sea monsters. With the dragons gone, that allowed the various non-dragon sea monsters from the tropics a chance to proliferate further and maybe grow to much larger sizes with more food for them available.
Given that the movie already has sea monsters, why not say that other fantasy creatures don't exist in the movie's universe? Given the announcement of the sequel, why not say that said sequel will be centered not around sea monsters like the first one, but around land creatures, making the series into full-on Fantasy Kitchen Sink. Not only this would provide a Worldbuilding of the world shown in the movie, but also provide a spotlight of gryffins who (excluding My Little Pony: Friendship Is Magic that portrayed them as sapient race) didn't had a interesting portrayal like dragons had in the How to Train Your Dragon and sea monsters in this movie, and the movie's creators own unique spin on dragons. As for the characters not acknowledging their presence in the first movie could be justified by the fact that those creatures aren't native to the area where the movie takes place, or at least being rare there.
