Life Of Siberian Reindeer Herders, Yamal, Russia.

"There is an ox shaped like a stag. In the middle of its forehead a single horn grows between its ears, taller and straighter than the animal horns with which we are familiar. At the top this horn spreads out like the palm of a hand or the branches of a tree. The females are of the same form as the males, and their horns are the same shape and size."
According to Olaus Magnus's Historia de Gentibus Septentrionalibus - printed in Rome in 1555 - Gustav I of Sweden sent 10 reindeer to Albert I, Duke of Prussia, in the year 1533. It may be these animals that Conrad Gessner had seen or heard of.