That is Gameshark. It is a client + server mod that only works if the server has it installed as well to prevent cheaters from using it. It allows the player to locate shinies, legendaries, bosses, and player-specified Pokemon from the Pixelmon mod.
If you're playing on a server, it looks like the server doesn't have it installed because it says "Unavailable". Some servers discourage Gameshark by preventing users who have it installed from logging in entirely or by banning them - so be careful. If you're using it in single player, I do know from experience that the Gameshark mod has been EXTREMELY buggy in recent updates which make it appear "Unavailable" - more specifically Pixelmon Reforged 7.0+.