My bettas' names (so far) have been Jaws, Blue, Jewel, and John Wick.
Jaws was a red veiltail and I thought it would be funny to name him after the movie Jaws since he's such a tiny fish (lol)
Blue was a big, mean butterfly who I named simply for his gorgeous deep blue color.
Jewel was named after Jewel Bundren from As I Lay Dying by William Faulkner; I'm not a huge fan of the character or anything, but his white body with turquoise-tipped fins made him look like a little jewel!
My last betta, who recently passed, was a dark blue double-tail male I called John Wick. I came up with his name long before I met him; I randomly thought, 'The next betta I get will be named John Wick.'