An elderly man, his death rapidly approaching, takes his young grandson up onto a hill behind his house and gives the boy his pocketwatch. Then, standing among falling apple blossoms, the man also "gives instruction" on the nature of time: how when you grow up, it begins to move faster and faster, slipping away from you in great chunks if you don't hold tightly onto it. Time is a pretty pony, with a wicked heart.
The story was originally a flashback scene to be included in a full-length novel-in-progress in which the grandson, a brutal hitman, reflects on his childhood. The novel was intended to be published under the Richard Bachman pseudonym.
My Pretty Pony is a phrase you use to count the time between seconds. It's referenced in Delores Claiborne twice when Delores is counting during a tense moment.
The audiobook version of this story, in the Nightmares and Dreamscapes collection, is read by Jerry Garcia.