Here is an arrangement of this song I have seen and the chords I have determined by ear. Hope they work for you.

(G) I'll hop up on my pony and I'll (Em) ride away
Down the (C) trail to (D) San An (G) tone.
(G) I can see her standin' waitin' by that (Em) garden gate,
And she (C)   waits for me a (D) lone.
(G) Through the hills and o'er the plains we'll (Em) ride, ride, ride.
(C)   I won't rest until I have her (D) by my side.
(G) Roll on, little pony. Let that (Em)   ol' wind moan.
I'm (C) headin' back to (D) San An (G) ton (Em) io,
(C) Down the trail to (D) San An (G) tone.
(G) I've just received a letter from my (Em) girl in San Antone
That (C) said she'd (D) be my (Em) bride.
(C) I know she really loves me and her (Em) heart is true,
(D) Guess I'll take myself another ride.
(G) Roll on, little pony. Let that (Em)   ol' wind moan.
I'm (C) headin' back to (D) San An (G) ton (Em) io,
(C) Down the trail to (D) San An (G) tone.
Instrumental
(G) Through the hills and o'er the plains we'll (Em) ride, ride, ride.
(C)   I won't rest until I have her (D) by my side.
(G) Roll on, little pony. Let that (Em)   ol' wind moan.
I'm (C) headin' back to (D) San An (G) ton (Em) io,
(C) Down the trail to (D) San An (G) tone.
Instrumental – Yodel
(C) Down the trail to (D) San An (G) tone.