He's seemingly at ease with Dick. It isn't entirely misleading. He's well within his comfort zone here, the man is obviously unarmed and the story he's given isn't one anyone who was lying would come up with. It's also one Peter knows is at least potentially true.

Mostly, he's very comfortable and confident in himself.

"Don't be ridiculous. You're a guest. The least I can do is give you the front end of the horse." He holds the animals reigns to steady it, but inclines his head for Dick to get on.

Also true, the quip about not giving the guest the horses ass but as comfortable and confident as he is, he wants no stranger that close to his back.