Enforcer Sentinel
From Halopedia, the Halo wiki
Summary
Enforcers are massive robotic automatons designed by the Forerunners that serve to contain major Flood outbreaks on Halo Installations.
The Enforcer is energy shielded, and can absorb more damage than a normal Sentinel. The shield, which only guards the front area of the machine, is immune to ballistic weapons, similar to the Jackal shield, but is weak against plasma weapons. It is also very strong, capable of withstanding three Scorpion shells and two Wraith shots.
An Enforcer is armed with rapid firing pulse beams similar to Needler fire, and mortar-like projectiles. Its mechanical grabbing arms can pick up and crush even heavy vehicles such as Scorpion Tanks if the driver is careless. However, by hopping out of the vehicle you can keep yourself and the vehicle alive.
They were only observed on Installation 05, most likely because Installation 05 was made "to research possible offensive and defensive measures against the Flood".
It seems likely that it wasn't created until after the demise of the Forerunner, and so they didn't transport it to the other Halos because they weren't around to do it.
