It seems the rules have changed recently. However, ponies may compete at senior (horse) events, but horses may not compete in pony events.
http://www.horseandhound.co.uk/forums/showthread.php?t=38354...
"Although the FEI's definition of a pony remains unchanged - 148cm without shoes or 149cm shod, a pony is permitted to stand a full two cm higher at competitions. "We have, however, taken into consideration the fact that measuring ponies at competition creates a different set of circumstances which may lead to fluctuation in height and following much consultation with the relevant technical committees we have allowed for a margin of 2cm," said the FEI spokesperson.
But the margin does not quell British Dressage's anxieties. "It is quite a small margin - from my experience a horse can grow more than two centimetres at a competition," the British Dressage spokesperson explained. "
http://www.horseandhound.co.uk/news/397/106440.html