Watching this video about the history of explicit fanart and image boards.
https://youtu.be/BYDr8jHYc4Y?t=1192 Starts mentioning boorus by name about here.
https://youtu.be/BYDr8jHYc4Y?t=1245 LOL, one of the oldest screenshots of e621?
He mentions DA/NG/Pixiv which were most popular early on in booru era, but are still going somewhat strong.

He doesn't have a furry-centric focus in this video but there's quite a lot of overlap with other boorus. Thought I'd share it as it goes over a LOT of stuff.