why were equilateral triangles spelled this way, or should it be 1+5 equils 6?
Probably because the english prefix “equi-” means equal, I’m guessing from the latin prefix aequi-.
Precisely. aequalis was one of several words derived from “aequi-”, and it’s the origin for “equal.”
It’s important for the OP to realize that the word equilateral breaks down to equi- (equal) and -lateral (side), not equil-ateral. That may be some of the source of the confusion.
And the word “lateral” means to do with sides. So “equilateral” means, literally, equal-sided.
Thanks…