People enjoying a river cruise in Queensland, Australia, got a creepy surprise when a strange creature emerged from the mud, leaving puzzled onlookers wondering what it is.

A video of the dark-coloured creature shows an ugly head sticking out of the wet mud with what looks like a wide mouth and a big eye, until it glides slowly back under the surface.

The curious footage - cut with dramatic music - was shared to the Facebook page of Solar Whisper Daintree River Crocodile & Wildlife Cruises. "Either a miniature whale, a mussel or an alien don't ask me???" said the post. "One minute he was there the next he was gone..."

Beneath the Facebook post, a debate raged about what the unidentified creature is. "Don't think it's a lungfish unless that's a very deep hole," wrote David Nye. "Looks more like a big mussel/pippy type thing."

Hasnah Binti Hussin wrote: "Underwater monster?"

Staff at the cruise company told the Daily Mail they believe the creature is actually a mudskipper, though they aren't certain. According to Encyclopedia Britannica, there are six species of muskipper, found in the Indo-Pacific.

"They live in swamps and estuaries and on mudflats and are noted for their ability to climb, walk, and skip about out of water," it says. "Elongated fishes, they range up to about 30 cm (12 inches) long...Their blunt heads are topped by large, movable, close-set, and protuberant eyes, and their strong pectoral fins aid them in movements on land."

Take a look for yourself.