Ok, I hope this is some help. If you're a Simpsons fan you may remember an episode in which that song is sung on the "Gabbo" show-if you're not a fan please stick with me.

The only lyrics in the show are "Old gray mare, she ain't what she used to be, ain't what she used to be, ain't what she used to be". To find the original artist and composer you could try looking at the credits at the end of that Simpsons episode as they are usually thorough with respect to that sort of thing. If I have that one on tape, I'll check it out and get back to yoy.