There is a photo of what is purported to be a Manx pony in "Legends of a Lifetime"by George E Quayle and reprinted in 1992.
There are sketches of ponies (pencil drawings done in 17th century) that are dragging sledges (sleds) in front of Castle Rushen. They can be found on the Manx Notebook.
It has been suggested to me that the Manx Pony was most similar to the Kerry Bog Pony which nearly became extinct.
If you Google "Kerry Bog Pony" you wil find a variety of photos.