Licid
Native to Rath. Licids are symbiotic, beetle-like creatures capable of snapping their bodies to jump several times their own body length to land on a host organism. Once latched on, the Licid's legs dissolve into the host and the two creatures bond together. The Licid then stimulates certain parts of the host's brain to bring certain emotions or instincts to extremes, and in some Licid sub-species, engender new physical attributes and abilities. Licids do not necessarily remain attached to their hosts for long and have been known to voluntarily separate from its victim to seek out another. Licids are very similar in appearance to the Xenomorph Face Huggers from the Aliens movies.
In the game, the Licids are renowned for being simple in concept but extremely complex in execution. They enter play as Creatures, but can transform into Auras, a type of card that must be attached to another, simulating their joining to a host. They can also transform back to creatures. It's a long-running joke in Wizards of the Coast R&D to ignore the fact that Licids ever existed and therefore make life much easier.
(Convulsing Licid, Transmogrifying Licid)
Read more about this topic: List Of Species In Magic: The Gathering