Ultimate Slim and All Terrain Slim
The following images show the difference in size between the regular Ultimates/All Terrains and the Ultimate/All Terrains Slims. In some of the sizes the height of the boots differs quite a bit and so can make the slim version a better choice if your horse is a finer build. The size 7 slim is tiny so will generally only fit very finely-built minis.
The Ultimate Slims have been re-desinged to have an increased built-in break over as well as a pre-marked spot for adding a drainage hole.