From Wikipedia:

Though oxen were used for most farm work into the 18th century, horses 'fit for the dray, the plough, or the chariot' were on sale at Smithfield Market in London as early as 1145.[8]

[8] Hart, E. (1986). The Book of the Heavy Horse. Wellingborough: Patrick Stephens Limited. pp. 45–63. ISBN 0-85059-640-8.