Pixelmon-style servers add catching, battling, breeding, and gyms inside Minecraft. You must install the matching modpack—the normal vanilla launcher will not connect.
Servers differ a lot on shop prices, legendary spawn rates, and ranked PvP. Using the wrong pack version is the most common join failure—match the listing exactly.
Many require Discord or a whitelist because modded support is heavy. Check that before you download a large pack.