I usually go by NightRaven, but that name was taken on this forum and I couldn't be bothered with numbers and underscores and things, so I replaced the Raven for Krow.
Night comes from my favourite band, Nightwish. Before I came up with the name NightRaven, I was DarkRaven, so Night is in keeping with the theme of darkness. Also night time is the best time of day because sleep XD
The Raven part in my usual username comes from the name that my dad goes by online - he is Crow or Crowsy most of the time. He likes the birds for their intelligence, as do I. I chose something in the same family as the crow, but different, just as I am in the same family as my dad, yet very, very different. (Also, Raven was a cool TV show when I was a kid. It's not on any more :( )
So, as I mentioned earlier, NightRaven was taken here, so I thought, as this is a Pokémon forum, I would replace Raven with the Pokémon that most closely resembles a raven. Thus, NightKrow was born.
My dad knows my PC name and told me that I can't be a crow because he's a crow, but I'm actually a krow so it's fine.