This was a "field expedient" - they cut down tons of small trees out of the local woods and sharpened the trunks. It is believed that they got the idea from the Ottoman Wars a couple of decades earlier.
But very few of the horses reached the English lines because of the massive volleys of arrows fired at the French knights. A lot of them tried to reach the English lines on foot but were bogged down in the mud and slain (or held for ransom).
To read all about the period in a nicely researched and charmingly written novel, see here:
Get the N.C. Wyeth illustrated version if you can. Look at this lineup of archers - rough crowd but good guys to have with you in a fight:
