Ralph's "nightly game of supposing" is the series of fantasies that Ralph imagines before he goes to sleep.
Doing this is both an act of self-comfort and an act of escape. When he imagines being somewhere else, somewhere peaceful, he can forget the tragic events happening all around him on the island.
He doesn't include "wild" ponies because something wild is too close to what is happening in real life. It wouldn't be an escapist fantasy if it reminded him too much of his real situation. He imagines things very different from what he is actually experiencing.
