Helioskrill

Helioskrills are predatory creatures that live on Sanghelios. In order to feed themselves, they remain completely still at a location, imitating a rock, waiting for unsuspecting prey to pass by.

Trivia

 * Their name, Helioskrills, suggests they are from Sanghelios even to those with limited info.
 * It is highly unlikely that the Sangheili are the "unsuspecting prey" the Helioskrills attack. The Sangheili surely are extremely more powerful than these predators, which would allow them to control their attacks.