Shielded Flood carrier form
These were tested by Bungie, and their unfinished forms have been found buried in the Halo 2 disc, but they have not been included in any actual game. The behavior of the Shielded Carrier Form is identical to the unshielded variant; the sole difference is that it is harder to shoot them and kill them.
Shielded Flood Carriers are part of the Deleted Material cut from the Halo game. When developing Halo 2, Bungie faced numerous delays for various technical reasons. The actual launch date of the game was almost a year after its original date.
As such, many aspects of the story such as vehicles, gameplay/design, and story elements did not make it into the final version of Halo 2. Many of the elements have been confirmed by videos and commentary on the Limited Edition special DVD for Halo 2.
Canonically this Flood form is possible, and in fact more than likely: Shields would indeed have proven very useful in getting deeper into enemy lines before exploding, as Carriers are used as suicidal deliverers of Pod infectors. It is impossible that the shields were simply assimilated into the Carrier Form from the fallen victims hand, as the shield gauntlets are attached securely to a Jackal's arm and the shield is located in the center of the repurposed corpse, not the arm. Thus, the assimilation of a Jackal Shield is probably intentional, as the Flood form would have to "consciously" transfer the gauntlet from the arm to the body. This level of strategic intelligence would surely only come about after the Flood reached its Coordinated Stage. But was likely removed from the game because of balancing issues.