"I supplied it with the Skins of Yahoos"
Skins - Credit: Kuerschner
This is the first of several passages that refer to Gulliver trapping, and skinning the Yahoos. He is totally alienated from himself, having associated his mental paradigm with the Houyhnhnms, and alienated from the Yahoos which are a reflection of the worst of humanity and therefore the worst in himself. With ease, he wears Yahoo skins.
The dicotomy of Gulliver coming to associate himself with the Houyhnhnms is that by skinning a creature very much like him, a human, would be a very Yahoo-like thing to do and the Houyhnhnms would despise him for the action.