The lion was legally shot dead by a hunter on 30 December.
The Idaho Department of Fish and Game says the teeth could be the remnants of a conjoined twin which died in the womb, or possibly a kind of tumour.
Biologists in the region have never seen anything like this particular deformity before, it says.
The lion was hunted after it attacked a dog near the town of Weston, Idaho. The dog survived, but a hunter set off in pursuit and killed the big cat a few hours later.
A conservation officer checked the lion's body, as required by law, and found the set of teeth, as well as what appeared to be a set of whiskers on the left side of the animal's forehead.
Mountain lions, a native species of big cat, are relatively common in Idaho. They are elusive animals which seldom come into contact with humans.
They are territorial and require large swathes of wilderness to thrive, according to the US National Wildlife Federation.
In winter, their prey species, such as deer and turkeys, move down from the mountains to escape colder temperatures, often gathering at the edges of villages or towns.
This can put predatory mountain lions into conflict with people, livestock and pets, says the Idaho Department of Fish and Game.
A mountain lion killed in the US state of Idaho had a set of fully formed teeth growing out of its forehead, wildlife officials say.