[NEW VERSION]
This is based off of Hypixel's nick system. The main command is /bookgui.
To unnick type /unnick.
https://www.spigotmc.org/resources/nick-plugin.96201/history



[OLD VERSION]
This plugin can be accessed by typing the following. If you want to Enter a name you type, /nick enter <name>, if you want to get a random name, type /nick random.To unnick you type /unnick. Here is the code of the plugin:


import org.bukkit.ChatColor;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.Locale;
import java.util.Random;

public final class Dfbgwadef extends JavaPlugin {

@Override
public void onEnable() {
// Plugin startup logic

}

@Override
public void onDisable() {
// Plugin shutdown logic
}

public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
if(sender.isOp() || sender.hasPermission("nick.gui")) {
if(command.getName().equalsIgnoreCase("nick")) {
if(args[0].equalsIgnoreCase("enter")) {
String Entername = args[1];
Player player = (Player) sender;
player.setDisplayName(Entername);
player.setPlayerListName(Entername);
return true;
}
else if(args[0].equalsIgnoreCase("random")) {
Player player = (Player) sender;
String ranNames = randomName();
player.setDisplayName(ranNames);
player.setPlayerListName(ranNames);
}

}

}
if(sender.isOp() || sender.hasPermission("unnick.me")) {
if(command.getName().equalsIgnoreCase("unnick")) {
Player player = (Player) sender;
String name = player.getName();
player.setDisplayName(name);
player.setPlayerListName(name);
return true;
}
}

return true;


}

public static String randomName() {
String[] name = {"ProGamer_yml1907", "PeriodGamer0193", "GamerHollow", "DoctorSpider", "HornSher_Gamer1902", "Soon_Yersh1239"};
String ranName = name[new Random().nextInt(name.length)];
return ranName;
}
}

Enjoy!

1.17.1 plugin.