500 is a sprite created by spriter of ye olde times, Nokrow889. As the name would suggest, he is Nok's 500th sprite. He's hideously deformed, and he loves to rape people and make people feel uncomfortable.
When he rapes, he does not get people pregant - rather, he gives them super AIDs, which will kill them. Him and DryBones88 have had kids in Nok's old series "Sprite School."
As of now, 500 is just an old meme. However, plenty of people have followed the 500 path with their sprites, trying to make them as famous and significant as 500, However, this has a similar affect that sprite series do - the first of its type is great, but everything else made by others fucking sucks. Examples of this include Choko, Steve, Dan, and plenty of 500 knock-offs (600, 400, etc etc).