I'm almost certain those are pigs. I'm having some trouble linking the two tests together, but pigs were certainly used in the Operation Plumbbob series of tests, though none of the Plumbbob detonations used the M65 Atomic Cannon, which was tested only once, as part of a series of tests called Upshot-Knothole.

Pigs were used during the Plumbbob tests because they're decent human analogues. Their organs are similarly sized and would have responded similarly to overpressure from the blast, and their skin reacts similarly to ours to burns from the thermal effects.