Although the Predator displays only three of his trophies onscreen, we witness the deaths of all six of Dutch's team members, leaving many wondering what happened to the remaining skulls. It's very likely the Predator collected trophies from all six victims, but we simply do not see this. The skull with the hole in it is said to be Poncho's, as he was shot (blasted) in the head. However, Poncho was shot in the left side of his head, while the hole is on the top of the skull, so this is either a goof or someone else. The others are unknown. One of them may have belonged to Billy, as we witness the Predator rip Billy's spine from his back and clean his skull, and the other was probably Dillon's as he put up the biggest fight besides Dutch and Billy. It has been speculated that one of the other skulls may have been Blain's since the Predator entered the squad's camp to retrieve his body.
Some believe the skull with the exit hole was Mac's because initially it appears that Mac is shot in the head. Also he is the only member of the team besides Dutch to injure the Predator. But in the brief shot of him laying on his back when Dillon finds him, you can clearly see he has a big hole in his chest, yet his head is unmarked. This is likely another continuity error in the movie, where the crew may had filmed a portion of this scene already and changed the setup for the second part, creating a goof. Some say that the angle from which he was shot created an exit wound through the back of his neck, and this, combined with the camera angle on his death, gives the appearance that the blood bursts from his head. However, when freeze-framing Mac's death scene, the blood can definitely be seen coming from Mac's forehead, not his neck. After his body falls down, his leg can be seen twitching, which also seems more compatible with brain trauma (because a shot through the chest and neck would likely sever the spinal cord, so the legs would no longer be able to receive nerve impulses from the brain). Also, it is possible that the three trophies that are displayed belonged to Jim Hopper's men. As we only see, three of Hopper's men strung up and skinned. Therefore the Predator may have kept Dutch's team mates' body parts somewhere else.