Mycoplasma pneumoniae

Etymology
Mycoplasma pneumoniae:

myco: "fungi" (from greek μύκης (mykes), meaning "fungus")

plasma: "formed" (from greek πλάσμα (plasma), meaning "formed" or "made")

pneumonia: "inflammation of the lungs" (from greek πνευμον, meaning "lung")

Explanation
Mycoplasma pneumoniae is an atypical bacteria, which lacks a rigid cell wall. While a bacteria, it has such unusual characteristics that it was initially assumed to be a fungus (myco), or at least in the form (plasma) of a fungus. It causes "walking pneumonia", which explains the second part of its name.