What really surprises the horse about Gulliver?

The horse was surprised by Gulliver's clothing, and the fact that he resituated his hat after it was disturbed.

The gray steed rubbed my hat all round with his right fore-hoof, and discomposed it so much that I was forced to adjust it better by taking it off and settling it again; whereat, both he and his companion (who was a brown bay) appeared to be much surprised: the latter felt the lappet of my coat, and finding it to hang loose about me, they both looked with new signs of wonder.

The horse was astounded by the following:

•Gulliver had a softer skin than Yahoos, and was more tamer than them, although he looked exactly like them, except for skin colour.

•Gulliver was a fast learner, in contrast to Yahoos, who were no better than animals.

•Gulliver wore clothes, which the horse thought of skin, but his doubts later cleared by Gulliver, that it was just a covering. Neither did Houyhmhmns, nor did Yahoos wear clothes.