There doesn't seem to be a way to pass a turn without moving. (That is, I couldn't find a "wait a turn" function.)
If you manage to turn yourself into a full horse, your Intelligence should totes drop sharply (maybe even get set to 1), hehe. Maybe your Willpower should drop too! (Being a horse seems pretty easy otherwise, since you can hit hard and eat grass to recover sanity.)
I would probably use 'spacebar' myself. But that means it's easier to whack the thing on accident, of course. (I use spacebar because I often play melee and want to wait for melee monsters to walk into my range.)
In NetHack, "rest on spacebar" is an optional setting, with period as default. So the period key works well!
Since s and x are both down, you could use one of those. I would say s makes most sense, then you've got the 8 directions plus the middle button, but people are so used to using wasd that it might be easier to get them to wait on x
You could also get people to use the numpad for movement, which would make it pretty intuitive. Not everyone has a numpad though
