Horsey Mill is not, of course, a mill at all; it's a wind pump. However, in the Broads they refer to all wind pumps as 'mills' (but never 'windmills') so Horsey Mill is its name.
Unless, of course, you're a thoroughly pretentious organisation like the National Trust in which case it's 'Horsey Windpump'.
The current building dates from 1912. It ceased operation in 1943 after it was struck by lightning.
The National Trust took ownership in 1948, and it was restored in 1961 by the Norfolk Windmills Trust and SPAB.
For details of opening times, admission prices, etc. please see the National Trust's web site.
There are paths down both sides of the dyke leading away from the mill. The one on the far side (to your left if you've got your back to the mill) leads to a view point over Horsey Mere.