A pet owner was shocked when they saw their greyhound sleeping in a frightening pose, and decided to enlist the internet's advice.

Reddit user Eli captured a slightly disturbing video of their dog with its eyes wide open, twitching, and baring its teeth, yet the catch was – the pup was asleep!

They had never seen the behavior from their four-legged friend, so they uploaded the clip to the group "WhatsWrongWithYourDog" on Reddit.

"My dog started doing this while he was dreaming. He's never bared his teeth before," Eli wrote.

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"Angry dream."

Several users agreed that the dog was having nightmares, and said they'd seen similar behavior in their own pets.

But can dogs really dream?

Indeed, most mammals do - but not all, as a study published in the Journal of Comparative Neurology in 2020 showed.

Among the animals that are not able to dream are – surprisingly – whales and dolphins, even though they are considered to be highly intelligent. Dogs, however, are said to be able to dream.

Accordingly, many Reddit users were certain that Eli had hit the mark with her guess.

"Typical greyhound sleep. mine does it all the time. the eyes are open, they twitch in a strange way. greyhounds are strange creatures," wrote one Reddit user.

Another came up with a rather creepy anecdote: "I have a retired racing greyhound and he I see him running while laying down as he's sleeping. I think he dreams about being on the racetrack again. Once he suddenly got up and ran down the hallway. I followed him and he was standing at the end of the hallway in the kitchen all confused."

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"This dog dreams in the most creepy way ever," wrote another user.

Eli may be lucky that her dog was merely baring his teeth in his sleep. It doesn't have to get any creepier than that.