Black dragon is the animal in mope.io game having the highest level. It is evolved from a dragon at 100,000,000 EXP. This animal can eat everything and it can’t be eaten by nothing. The place that a black dragon can hide includes tall grass. There is not any animal after this dragon.
If you visit the deobfuscated mope.io source code at http://pastebin.com/tnMMsasm, you should find the line 355. It shows the drawAnimal() function. In which, it lists the HTML color codes for each animal. You can see the color code for “???” (the animal after Dragon) is 000000, or pure black/transparent.
At the line 621, you will see that the switch () function gives you the appropriate message when you transit to another animal. For “???”, it reads “Upgraded!” Unknown may be the black dragon. In fact, it’s impossible to take 100,000,000 EXP.
And thus, the animal after the dragon in mope.io online does not have a hard-coded name or upgrade message. This unknown animal has the same abilities as the dragon. But, its skin is not as same as the dragon. So, it is called Black Dragon.