Pixelmon in Minecraft is a trendy mod that adds Pokemon to the game. While Pixelmon can easily be played alone in a regular survival world, lots of players enjoy playing it on a server with others.
PokeSaga is one of the smaller servers featured on this list. There are only about 400 players online on average at any given time. The store for PokeSaga is more fleshed out than most of the others ...
The Minecraft mod launcher PolyMC might not be something you've personally heard of, but a lot of people use it to play the sandbox game if they have multiple game profiles and different mod packs.