Donkey Mong is an ultra rare monster that spawns in the Bullymong areas of Eridium Blight. You will enter this area automatically during a story mission. He looks like one of the common Bedrock Bullymogs, but throws explosive barrels at you.

Donkey Mong has only one single spawn point, however, he spawns very rarely. He can also spawn at any time of day. Take note that you do not have to do any special requirements to make him spawn.

Here's a video to help (NOT my video):