Five O'Clock Charlie — Marguerite Henry

Charlie's not old! So what if he's 28 and most people consider that a little old for a horse. When his rheumatism isn't acting up, he's as frisky as any young colt. And he's certainly not ready for retirement. Charlie can tackle any tough job that comes his way.
Unfortunately, Mr. Spinks, Charlie's owner, doesn't quite agree. So he makes Charlie take a permanent, though well-deserved, vacation. Poor Charlie is bored to death!
But then Charlie discovers there is something he can do. Maybe retirement isn't so boring after all!?!