I tried to find its origins last year, but it's in the void now. I've forgotten why I chose Haichou as my username ^^;
What I do remember is writing down name ideas as fast as I could because I was on the signup page for SoundCloud and hadn't thought that far ahead. Most of them ended in "chou," and I know one of them was Ikachou: squid + butterfly. The second half of my name comes from that.
I think I plugged in random syllables until I got "hai" and said "sure, that'll work."
I used to go by Mizuiro16, too. 16 was a meaningful number to me, and I just liked the way "mizuiro" sounded.