Pulse beam Sentinels
From Halopedia, the Halo wiki
The pulse beam Sentinels are a deleted variant of Sentinel, once intended for Halo 2. Similar to Enforcers, these Sentinels used pulse beams instead of Sentinel Beams. When destroyed, they would drop pulse beam weapons identical in appearance to Sentinel beams, but with a unique targeting reticle, and they would fire red needles.
While the Needler Sentinel superficially resembles the pulse beam Sentinel, it is a glitch distinctly different.