Here's two examples from the Borgia codex. Page 3 (Panel 5) and page 6 (Panel 3). There's also lots of examples in the Mendoza codex. I guess it's some sort of display of dominance and control, but is there anything more to it than that?
In Mesoamerica it signifies that someone captured a prisoner. It was a sign of defeat and shame to hold another warrior by their hair and hold their head back. This a scene is from the Bonampak murals in the Maya region. As you can see, the Maya also depict prisoners in the same fashion. Even in West Mexico, the shaft tomb culture depicted the same sort of position with a warrior holding a prisoner by the hair
