What follows is a list of Friendship is Magic based MUDs, MUSHs, MUCKs, and whatever other MU*s show around.

Recommended

Equestrian Dawn @ mush.equestriandawn.com Port 8789
EquestriaMUCK @ equestria.kaitain.org Port 2700
Role Play is Magic @ heartbeat.strangled.net Port 7076
PonyMUSH @ game.ponymush.com Port 3080
(NSFW) Discordia MUCK @ saerydoth.net Port 8888
(NSFW) Neighvada Nights @ neighvadanights.com Port 8888

Tread Carefully

MLP FiM MUSH - Warning: Autoplaying music on webpage.

EquestriaMUSH - You are required to be a member of the Awesome Ponies forums to join this MUSH. Previously, an In-Character Pony Tumblr was required as well. I was never able to join.

Also check mudstats.com for more info about MLP FiM MU*s.