"[Whoever] challenges you to a fight in the Battle Coliseum!" but then they don't show in my list here: http://subeta.net/games/battle/challenge.php

It has happened for both the Ice King and for Twisted Jaxon. Am I missing something here? Is there some other list of challengers?

It shouldn't happen too often. Ice King is a special challenger that appears in a random event. From the time of the random event you have 10 or 15 minutes to beat him then he disappears until the next event. Twisted Jaxon was available up until a few hours ago but has since been defeated. He was a boss that everyone fought and started with 200 million health.

:( Well that's not fair. I just got online. :(

And thanks for the info on the Ice King. I didn't realize he disappeared from the list. (Although, I was confused why he kept challenging me.)

The timing was unfortunate for a lot of people, I know I was stuck at work when he first came out then had to rush off to dinner straight after. Stayed up until 2am just to get some battles in. They tried to make it as fair as possible, he was available for 24 hours. It won't be the last big boss fight, this is the 3rd one this year and I'm sure there will be more in the future :)

Boo. :( When I wasn't working I was catching up on sleep. Never got a chance to touch this guy.