Harvester

Harvesters are huge insectoid-reptile creatures that inhabit several worlds throughout the galaxy, including Tuchanka and Tarith.

Harvesters are intimidating to say the least. Nearly the size of the visible part of a Thresher Maw and equally as aggressive, their primary means of attack appears to be flying low and "dropping" klixen. This could mean that harvesters either birth klixen or simply pick them up and relocate them. Either way suggests a rather fiendish intelligence; the harvesters are smart enough to use klixen both as shock troops and, failing that, mobile ordnance.

Trivia

 * Harvesters were first described in The Art of Mass Effect. They were intended to be in the original game as an enemy that could only be killed in the Mako; however, they were excluded due to time and budget constraints. They were later added in Mass Effect 2.
 * In Mass Effect, on planet Maji in the Vamshi cluster in the Armstrong Nebula, you can find an odd skull which resembles a harvester.