In "It's the Scary Old Custodian, Adam Lyon", Old Joe was lurking in the school Boiler Room, when Adam and Jake went down there. Old Joe was a lone human being in CDMS, mopping the floors in the dark basement, where no one could see him. This got Adam to connect with him, as he was suffering from being the only human student at his school. Old Joe explained why a human was working at CDMS. He said that long ago, Charles Darwin Middle School was a regular human school. One day, the school became integrated and later, completely dominated by animal faculty and students. However, he hid out in the basement and became the only remaining human, who wasn't replaced by by the animals.
In the end, it turned out Old Joe was actually just Principal Pixiefrog and Mrs. Warthog dressed up in costume, messing with Adam and Jake. This prank managed to scare them away, as they sat there, laughing at them. Just then, in the final twist of the ending, the real Old Joe entered, only now, he was apparently the ghost of his former self. He yelled at Pixiefrog and Warthog for making a mess on the floor, with their costume bits, and the two ran off, screaming, while he broomed the clothing off the floor with his ghost broom.