The animal died shortly after birth
© Pen News/Heather Herman
The animal died shortly after birth
Heather Herman made the discovery when inspecting the livestock at the family ranch in Montana.

The 30-year-old was looking after her herd when she discovered the two-headed creature amongst the newborns.

"I was doing one of my daily checks through our herd and noticed a cow alone so I went to check on her," she said.

"The calf was laying next to her but from a distance I could tell something wasn't quite right.

"I had very mixed reactions when I realised what was wrong."

Two-headed creatures are rare, especially among mammals, which have fewer offspring than, for example, reptiles.

They can occur when an embryo splits, but not completely - leaving the creature with two heads.

It can also occur when two separated embryos incompletely fuse in the womb.

Two two-headed cows were born just days apart in India earlier this year - but the mutations rarely survive to adulthood.

Heather went on: "I honestly didn't want to look at it when I realized what it was."

She said the farm had been in the family for seven generations, but there was only one other incident like this they could recall.

"It's pretty rare, I've seen pictures before but have never seen it happen in person," she said.

"Talking with my grandma she had seen a two-headed calf one other time. No one else I've talked to had ever seen it.

Justin Adams of Monash University in Melbourne, Australia, said mammalian bodies tended to undertake a sort of genetic fact-checking that prevented such embryos implementing.

"If there are errors in development, there are miscarriages and spontaneous abortions," he said.

Heather added: "We were amazed the mother was able to have the calf unassisted, we could have easily lost the cow.

"She was fine and didn't seem to have any issues calving. She was a mature cow who had had at least four normal calves before."