Xenocite

The Xenocites are alien "leeches" artificially manufactured and mass-produced by the Highbreed that create DNAliens when attached to a host.

Reproduction
Xenocites eggs are created from an egg-laying machine. The eggs hatch underwater in large artificial rivers (that look similar to the pools used to store radioactive material). After hatching, the Xenocites are stored in boxes filled with DNAlien slime, probably to keep them fresh.

Powers and abilities
As they attach to their host, Xenocites replace the characteristics of the host species with standard DNAlien abilities. Within hours, the Xenocite will reprogram the host's mind and completely cover their body, transforming them into a DNAlien.

Xenocites possess enhanced strength and agility that they use to avoid attacks and quickly latch on to a victim.

Considering that they were seen living in open air and many were shoved into wooden crates with seemingly no DNAlien slime, it is implied that Xenocites are amphibious lifeforms.

Weaknesses
Xenocites are unable to merge with silicon-based lifeforms, such as a Crystalsapien.

The Omnitrix's genetic repair function can separate a Xenocite from its host, thus negating the DNAlien transformation.

The DNA Repair Guns created by Cooper Daniels can destroy Xenocites, turning the hosts back into Humans.