Hi there! I'm the one who originally made the challenge due to the request of followers of my FuckYeahMonsterGirls blog over on Tumblr.
The concept of a "30 Day Challenge" isn't new or exclusive to this challenge at all-- I simply took the formula of drawing 1 thing according to a prompt every day and made it relevant to Monster Girls, so that's why it's called the "30 Day Monster Girl Challenge". Many others have made sequel lists or revised my list, but from my knowledge my list is the first to get "popular".
Which I would say is fairly is? There are new drawings in the 30 Day Monster Girl Challenge on Tumblr everyday and we get quite the few submissions here!