Pixelmon is, in my opinion, the most unique game mode in Minecraft. While in other modes you might be mining deep to find those diamonds and make yourself some epic gear, fighting others in the battle royale of the hunger games or competing in the latest skyblock challenge, pixelmon on the other hand has you replay the all too nostalgic Pokemon series in Minecraft. There are thousands and thousands of wild pokemon simply roaming the realms, with every wild animal that's usually running around being replaced by them. you can also obviously catch and train them and eventually use them to battle with others, some servers even have NPCs against which you can challenge yourself and measure your strength.
There are plenty of Minecraft servers dedicated only to this mode, with many of them being heavily curated to deliver you the best experience. Some are even recreations of the older pokemon games, such as the red and blue editions or the silver and gold, while others craft a completely new experience. Most every Minecraft server list, has at least a pixelmon filter if not a whole category devoted to it, that should tell you enough about the popularity of this game mode.
Lastly I'd like to mention the pixelspark modpack, which is pretty much the modpack for you, if you want to play this game mode. It basically changes the server from a Minecraft server with a mod to a completely new game. It gives a whole new experience to your pixelmon server. So dont waste anymore time and find a list of pixelmon servers now !