A destrier is a heavy war horse. It was bred and trained for battle in the Middle Ages, and might have been heavier and slower but longer-lasting than a charger or steed. A knight might reserve the destrier for battle and travel from place to place on a palfrey or walking horse.

It comes from the Latin dextrarius, via Old French, meaning (led) by the right hand, that is led by a squire (dexter = right).

As an example quotation, Chaucer uses the word in his The Tale of Sir Thopas, a very dull piece of doggerel that Chaucer himself recites when called upon by the Host of their Pilgrimage. This verse occurs just before the Host interrupts Chaucer for being tedious and demands something better. Here's one version1:

And, for he was a knight auntrous,
He woulde sleepen in none house,
But liggen in his hood,
His brighte helm was his wanger2,
And by him baited his destrer
Of herbes fine and good.
And here's how it appears in the E2 node:
And for he was a knyght auntrous,
He nolde slepen in noon hous,
   But liggen in his hoode.
His brighte helm was his wonger,
And by hym baiteth his dextrer
   Of herbes fyne and goode.
Or in modern English,
And because he was an adventurous knight,
He wouldn't sleep in any house,
But lay in his hood.
His bright helm was his pillow,
And he fed his destrier himself
With fine and good grass.
1. From http://www.literatureproject.com/canterbury-tales/canterbury-tales_18.htm
2. Wanger, a pillow, from an old word for cheek. Why, what were you thinking, missus?