Effigies of armoured knights, cross-legged, with their feet on some animal or other, are common. This one is well-abraded by time. There are plenty of better examples.
Only this one is tiny- the size of a large doll- and that is very unusual.
The explanation is that it's a heart shrine. The knight died abroad- probably while off on the crusades- and his heart was preserved and shipped home to be buried in his parish church. There was no full sized coffin, just a casket, and the effigy was made to its measure. It occupies a wall niche in the chancel, to the left of the altar.
We don't know who the knight was but almost certainly one of the De Cahagnes family- who were given the village of Horsted by William the Conqueror- and whose name (in a variant spelling) eventually got attached to it.