James Dyer (in ‘Southern England: an archaeological guide’) suggests the stone resembles a horse. Methinks he has a good imagination, but perhaps I’m missing the point.
He says that not only are there fragments of stone in the vicinity, which suggest the stone was part of a tomb, but (drawing on ‘South East England’ by R Jessup) that another burial chamber used to be nearby too.
It was “similar to the Chestnuts and Coldrum, and was discovered by a ploughman at the other end of the same field in 1823. It seems to have consisted of a tomb 2.1m long, with three wall stones, and to have contained human bones and pottery. Known as Smythe’s megalith, after the antiquary who best recorded it, it has since been destroyed.”