I'm assuming you're referring to "The Galloping Foxley" by Roald Dahl.

"The Galloping Foxley" is a short story by Roald Dahl, first published in 1946. The story revolves around a man named Mr. Foxley, who suffers from a rare and embarrassing condition where he involuntarily gallops like a horse whenever he's nervous or anxious.

Would you like to know more about the story, or are you looking for a PDF version of it?