The unicorn is a mythical creature often featured in children's fairy tales, but a South Australian family now has the next best thing living in their own backyard.

Key points: Joey the ram has a horn growing straight up from the top of its head

Joey the ram has a horn growing straight up from the top of its head A stock agent says he purchased the rare ram for just two cartons of beer

A stock agent says he purchased the rare ram for just two cartons of beer An animal expert says he has never seen a sheep or ram like this before

Stock agent Michael Foster spotted Joey the "unicorn sheep" out at one of his client's properties near Hallett, in the state's Mid North region, in February.

Joey was destined to become food, but the rare deformity of a single horn growing out the middle of his head, helped save his life.

Mr Foster said Joey was only hours away from being taken away, before he decided to throw the unique animal a lifeline.

"He was just running with the mob there and he just stood out to me," he said.

"He was booked to go to a feedlot with his brothers and sisters and we sort of picked him up the night before he was meant to go and saved him from that."

And it only cost the family a couple of cartons of beer.

"He cost two cartons of beer, it was two cartons of Great Northern," he said.

"I put the deal to [the farmer] and he said 'don't worry about it, just buy me a beer', but I thought he was worth more than that, so I said 'I'll give you two cartons instead'."

'Unicorn rides' at birthday parties

The Foster family now plans to keep Joey, named by his youngest daughter, as a pet on their property in Burra, take him to agricultural shows and pageants and maybe even to birthday parties.

"The kids would love unicorn rides," Mr Foster said.

"I've got two daughters and they absolutely love him, they reckon it's pretty cool that dad has a real-life unicorn."

The ram was named Joey by Mr Foster's youngest daughter. ( Supplied )

Mr Foster said he had spoken to many sheep farmers in the region who said they had never seen anything like Joey before.

"My guess is that it's some sort of genetic deformity … since the horn started growing they noticed it and it's grown from there," he said.

"I know guys who have been breeding sheep all their lives, I spoke to them and they certainly haven't seen anything like it."

Animal expert says 'it's pretty unique'

University of Adelaide Davies Research Centre animal scientist John Williams said he also had never seen a sheep like this one before.

Joey was purchased by Mr Foster for a price of just two cartons of beer. ( Supplied )

He said to understand the deformity you had to look into how the horns formed.

"This happens where the cells grow from what we call the neural crest in the embryo up onto the head of the animal," he said.

"Then they form horn buds and then they grow out into the horn.

"One could imagine that somewhere through embryonic development, something has gone a little bit awry with where these cells have come from and you've ended up with this one horn on the middle of the head.

"It's pretty unique."

Professor Williams said he had heard of many horn deformities before, but had not seen a horn grow naturally in this way.

"How many others are like this in Australia? I wouldn't like to hazard a guess, probably not very many, if any at all," he said.

"I've never seen one transpose like this naturally onto the middle of the head of an animal before."