The Money Dragon is a character who appears as the main antagonist during Mr. Krabs' imagination scene in the episode "Safe Deposit Krabs."
The Money Dragon greatly resembles an eel. It is light green in color and seems to love eating money by sucking it into it's mouth. The eel's eye color is yellow with red pupils. Instead of fire, money is spurted out of its mouth, hence the name. The Money Dragon also has a long, pink tongue and white jagged teeth.
The Money Dragon is seen when it "eats" Mrs. Greenback. Mr. Krabs furiously attacks the dragon and eventually "destroys" it. The Money Dragon, in reality, is actually a vacuum cleaner.
Even though it is called a Money Dragon, it resembles an eel.
It's possible that Mr. Krabs mistook the eel for a dragon.