Zo'op-pattern Weevil

Were you looking for the Covenant Type-27 "Mantis" or Type-38 "Tyrant" AA cannons or the Unidentified Covenant artillery?

The Covenant artillery is a ground-based Covenant field artillery piece. It fires bolts of plasma in indirect parabolic trajectories, and is essentially a larger version of the plasma mortar mounted on Wraiths. It is remarkably similar to the Covenant Mega Turret, albeit smaller.

Halo 2 E3 Demo
The Covenant artillery gun was first seen in the Halo 2 E3 Demo, where one such emplacement was placed on top of a building in New Mombasa, used by the Covenant to demolish buildings in the city. Early in the demo, the artillery piece is encountered by SPARTAN John-117 and a group of Marines, and Sergeant Banks calls in an air strike to destroy it. The artillery battery is much larger in the demo than its retail version counterpart.

Halo 2
The roles of the artillery guns as an active part of the Covenant assault were greatly downgraded in the retail version of Halo 2, as the levels featuring the guns were completely redesigned. Several artillery pieces are deployed on the beaches of Old Mombasa in the level Outskirts, but none of them are seen being fired during gameplay.