Check it out ... the woman barely has time to pose for a pic with the horse outside the Household Cavalry Museum when it chomps down on her arm -- prompting her to jump away and yell out in pain. The situation escalated when the tourist went down to the ground in pain, with police arriving to check on her.

While the incident is an unfortunate one, the woman was warned about the risk of injury ... in fact, you can peep the beware message in the video.

Watch the clip ... you can see a sign that reads "BEWARE: Horses may kick or bite," directly next to the horse and his guard.

This isn't the first time one of the horses in the King's stable has had an unfortunate run-in with a tourist. For those unfamiliar, it's a big no-no to touch the animal or the watchman ... though, that hasn't stopped a number of tourists from trying to grab onto the reins.

However, this recent incident appears to be just an unfortunate situation ... as the woman was a respectful distance from the steed.