Shellhead's Species

Shellhead's Species is an unnamed race of turtle-like aliens.

Appearance
According to Ken Tennyson, this species resemble giant turtles.

Powers
According to Ken, this species has the ability to hide in their shells whenever they feel threatened.

Omnitrix Transformation
The Omnitrix's representative for this species is named Shellhead, who was confirmed to exist in the original series episode "Ken 10". According to Derrick J. Wyatt, his DNA is currently not present in the Omnitrix as of Omniverse.

Shellhead has been used by Ben 10,000 (albeit off-screen).