"The Dance" was originally recorded by its writer, Tony Arata.
"Friends in Low Places" was recorded by Mark Chesnutt around the same time as Garth's version (though Garth did do the demo, the last one he'd ever have to make). Chesnutt's version was relegated to a B-side, though.
"Two of a Kind, Workin' on a Full House" is another song which was previously a single for one of its writers; Dennis Robbins in this case.
"Shameless" and "To Make You Feel My Love" were both semi-hits for Billy Joel first (and the latter's a Bob Dylan cover).
"Callin' Baton Rouge" was first recorded by The Oak Ridge Boys in 1978 and then a minor hit for New Grass Revival. Garth managed to zig-zag this trope by having New Grass Revival play on his version.
Two of his duets with Trisha were originally recorded by other artists: "Squeeze Me In" by its writer, Delbert McClinton (Lee Roy Parnell also cut it before Garth got to it), and "Love Will Always Win" by Faith Hill.
Subverted with his cover of Aerosmith's "Fever", which he rewrote extensively to make it a song about a rodeo rider. Oh yeah, and the fact that his version of the song barely charted.
"Unanswered Prayers" which has the narrator thanking God for ignoring his requests to let him marry his high school sweetheart, because if he had he never would have met his current wife.