Life Of Siberian Reindeer Herders, Yamal, Russia.

Both Aristotle and Theophrastus have short accounts - probably based on the same source - of an ox-sized deer species, named tarandos, living in the land of the Bodines in Scythia, which was able to change the colour of its fur to obtain camouflage. The latter is probably a misunderstanding of the seasonal change in reindeer fur colour. The descriptions have been interpreted as being of reindeer living in the southern Ural Mountains at c. 350 BC
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.