Lava eels were serpent-like Rahi of Ta-Metru. When agitated, the surface temperature of a lava eel's skin increased to the point where it could melt through metal.1, 2 Ta-Matoran often kept small lava eels as pets1 until they grew larger and more destructive, at which point they were normally abandoned.2
Notes
Lava eels could be seen coiled around molten protodermis pipes in the Fikou web (probably under Ta-Metru). They were long, black serpentlike creatures. In Ta-Metru, sometimes Matoran found small ones near the forges and brought them home. Then they got too big and destructive, so they got abandoned. Some lurked around the furnaces, some hid in the reclamation yards, and some apparently wound up in the Fikou web. The surface of their bodies was extremely hot, able to reduce things to ash and leaving scorch marks wherever they slithered. Lava eels were not by nature hostile creatures, as long as nothing agitated them. When agitated, they would hiss and squirm, and their bodies heated up rapidly. When agitated, their bodies could melt through stone.3
Lava eels were serpents living in Ta-Metru which thrived on heat. They loved to be near molten protodermis pools. When they were little, they were often adopted by Ta-Matoran as pets, then abandoned when they got too large and destructive. When agitated, a lava eel's skin heated up, and the creature could melt through solid rock.4
Lava eels gradually grew bigger.5
Lava eels and venom eels likely weren't related.6