Greatly to my surprise, this is a common word, in the 4,000s of our frequency list.

a. The male of the deer, esp. of the red deer; a stag; spec. a male deer after its fifth year.

• 07.133 Sometimes Bilbo saw the horns of the harts
• 08.030 when the hart bore down on him,
• 08.032 and look for the hart,
• 08.033 a hind and fawns as snowy white as the hart had been dark.

“hart, n.” OED Online. Oxford University Press, June 2016. Web. 28 June 2016.