Now I'm curious. what is it?
While the horse probably has teeth, where are they? One of the things horses do when agitated is show their teeth. Horses also pin their ears back when upset or angry.

This horse is having his mouth examined. Notice the vet is holding the horses tongue to the side.

At least the horse appears to be rolling his eyes. Here's what this might mean; Horses roll their eyes mainly to see what is going on beside or behind them, or to let you know they may be a bit nervous about something going on (sometimes looking for an escape route, sometimes as a bit of a warning).