Life Of Siberian Reindeer Herders, Yamal, Russia.

A deer-like animal described by Julius Caesar in his Commentarii de Bello Gallico (chapter 6.26) from the Hercynian Forest in the year 53 BC is most certainly to be interpreted as reindeer:
"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.