Today I was checking my server list and hovered over a player count on HiveMC and saw this:
Does anyone know, how to do that on a BungeeCord server?
I believe you are looking for the ServerInfo plugin created by joehot200. In the instance of HiveMC, they have used custom symbols to create that logo, but it was probably done with either this plugin or their own privately coded version. You can see the resource page for ServerInfo here.
By using this plugin, you can customize the text and color of what appears when a user mouses over your number of players connected.