Sit down before you read this shit. If it doesn't blow your mind it will certainly loosen your teeth and make your knees tremble a bit.
Apparently in Middle English, der (Old English dēor) meant a wild animal of any kind. This was as opposed to cattle, which then meant any sort of domestic livestock that was easy to collect and remove from the land related to modern chattel (property).
All modern Germanic languages save English and Scots retain the more general sense: for example, German Tier, Alemannic Diere or Tiere, Pennsylvania Dutch Gedier, Dutch dier, Afrikaans dier, Limburgish diere, Norwegian dyr, Swedish djur, Danish dyr, Icelandic dýr, Faroese dýr, West Frisian dier, and North Frisian diarten, all of which mean "animal".
A castrated male deer is called a havier.
Check out this little guy.