Back in my hacker-in-training days at college / Uni, it became apparant that "Alan" wouldn't cut it as an identifier because there was lots of them. I needed a unique alias.
Searching for ideas, I watched Top Gun and there was a character in there called Cougar. I kinda liked the name, it's sort of a pun on my surname and I'm a 'cat person' so it seemed like a good idea. Twenty years on, it appears to have stuck somewhat. People who've known me initially online call me Cougar in preference to my real name, so it doesn't seem to be going anywhere anytime soon.
In the days of the Web it's no longer unique, so I go with Cougar when it's available and UKCougar when it's not.