The Parasite, also known as the Sucker, is an enemy from Final Fantasy II. Its attacks, while relatively weak, drain MP. This can actually be towards the player's benefit, if they want to gain MP levels. They are otherwise not a big threat and can be easily disposed of with physical attacks.
Parasitism is a non-mutual relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host.