Energy leeches are nasty little nightcrawlers which are exactly what their name describes: giant leeches that feed on energy. They are known to inhabit planets populated by sentient robots, as they are quite good at sucking energon directly from the circuits of unsuspecting robot life, and are generally found in underground caverns and corridors.
The Transformers cartoon
The energy leeches were first encountered on the planet Chaar. They lived in the dark depths of the planet's caverns, dropping down from the ceiling and sucking the energon directly from the circuits of Optimus Prime, Goldbug and others. The Autobots were rescued by Galvatron, bizarrely-enough.
The Headmasters cartoon
When Optimus Prime was on a quest to locate the chamber of Vector Sigma deep within the bowels of Cybertron, he came upon an ancient room glowing strangely. The glow was actually from the energy leeches, which once again attached themselves to his body and began to drain him of all his energon. Optimus quickly swatted them off and ran away as fast as possible.