Most of the sites listed here are for MLP content, but there are varying degrees of overlapping in what they and this Booru allow.
I heard of an MLP Booru called Rainbooru which apparently has its own engine, but it’s down, I never got to see it, I can’t even find any screenshots besides a couple posted on Facebook (which is blocked in my country and I can’t register on it without revealing my identity).
booru.foalcon.com hosts humanised young MLP characters. I can’t remember if it has anthros and I can’t visit it because it’s having issues at the time (I HOPE it’s not dead). I used to be able to see it, but only with a VPN. It’s based on the Szurubooru engine, which I don’t know much about, and I only know 2 Boorus based on it: this one and a demo one.
Twibooru is one of those Boorus that are very similar to Derpibooru, but its Cloudflare now blocks me regardless if I use a VPN or not. IIRC it’s less restrictive than Derpi, allowing lolisho.
Baraag and Aethy are instances in Fediverse that are similar in how they both allow young NSFW art, whether it’s furry or human, but they’re uncomfortable to view because of the claustrophobic X-like format. I think reposting their content here would make it a lot more comfortable to browse it.
I made this Excel table comparing criterias of a few Mastodon instances that may be of interest to this site. Please note I made it without feedback, I don’t guarantee everything I wrote is correct. I avoid general furry instances because there may be too many and they’re most likely anti-cub.