Pixelmon is one of the most popular game modes on Minecraft and with good reason. It’s a fusion of Minecraft mechanics and gameplay with a Pokemon world letting you catch, train and battle with your pokemon in a Minecraft world. Most of the servers simply replaced the random animals of the Minecraft world with the assorted Pokemon, obviously some might be rarer than others and there are different pokemon in different generations of Pokemon games so you can take your pick of the server, depending on which part of the Pokemon universe you’d like to experience in your Minecraft game.
Personally I enjoy the ruby and sapphire generation of Pokemon the most and have been playing a lot on a server that focuses on this era. While there are also other servers that I tried out, I find these have the most lived-in feeling to them, letting me relive the nostalgic feeling of starting the game with a single pokemon and climbing up, training my pokemon going so far as to have a whole roster of max level beasts, letting you run amok other people showing off your time and dedication to them. There are obviously also people who put their time in devising strategies and battling other players.
Most servers also run events, where there’s a small chance a rare and truly powerful, sometimes even legendary, pokemon will appear in the wild and players will be driven to frenzy trying to find and capture it, before someone else does. All in all, I think pixelmon servers are some of the most innovative and fun servers offered on Minecraft.