Minecraft has a lot of different diverging ways of playing the game. Some people enjoy playing the game the way Mojang intended it, digging deep down to find the riches earth has to offer and possibly hit lava and die, others prefer to play with others in different challenges like the skyblock challenge or in teams such as factions, however one of the more innovative ways to play the game has to be the pixelmon mod for Minecraft. It offers a complete overhaul of the way you approach the game. No longer do you dig deep to find diamonds, no longer do you walk in fear of endermen, trying to farm them for their pearls, instead you walk around the world finding the ambient animals and other enemies replaced by a bevy of Pokemon of various generations, ready to be caught and battled with.
Now obviously if you're going to talk about Pokemon related games you have to mention the nostalgia factor. Plenty of people played their first Pokemon game years and even decades ago and are more than willing to relive that experience, especially if its intermingled with their favorite new game. Others might simply want to check out what all the buzz is about and get hooked when they play. It's a really fun and new way to enjoy Minecraft.
Most Minecraft servers that offer this mod are pretty much dedicated to it specifically as it's quite a unique experience when compared to vanilla Minecraft. However a few of the larger servers also started offering a realm with this mod because of it's popularity. There are plenty of Minecraft server lists that are already featuring a special filter just for this mod, showing you just how fun and demanded it is. However if you're looking for a specific recommendation, I'd tell you to look for Pixelmon servers on this link.