You have to play a mini-game to calm him down and obtain him. He was originally from Rusty's Ranch on Dusty Gultch. He is brown with tan spots and has a brown mane and tail. You must use him to ride around different cities and deliver the letter to the Marshal Taylor and also you use him to chase El Mustachio.
Elmer in The Smurfs ad.
Elmer can be seen in this poster for Back Lot Island
Elmer appears in The Smurfs (Movie) Ad, in the building, where you can see him with blue paint covering his face, dripping down his neck, and wearing a smurf hat.
If you haven't had tamed him, his old owner said that he would have been heading to the glue factory for good, which could be a reference the glue company Elmer, as well as the process of sending animals to the glue factory to be made and used as animal glue in the 18th and 19th century.
If you have a Follower when you get onto Elmer, the Follower will stop following you and hover in the place you mounted Elmer. When you dismount, your follower will suddenly appear next to you. This is kind of like the Mech on Steamworks Island.