Rival #3: Oakland Athletics
This is another rivalary that hasn’t yet taken off. In a way it is similar to the Angels situation. There just isn’t much of history to drive this one.
However, you do have the connection with players who have played for both clubs. The most well-known example is Jed Lowrie. Not only has he been traded from the Astros to the Athletics once but twice. Houston at one point had Chris Carter, Max Stassi and Brad Peacock on the active roster, albeit for a short period of time. They all came over from Oakland in the original Lowrie trade following the 2012 season. But Carter is long gone nowadays and Stassi is toiling away in the minor leagues. And Peacock has been converted (for good?) from a starting pitcher to a major league reliever.
However, there is one of the current Astros, Josh Reddick, who made his career by playing in Oakland. The former Athletic outfielder was, of course, traded last summer to the Los Angeles Dodgers and then signed with the Astros during the offseason. He obviously holds fond memories in Oakland and his past teammates so this adds another wrinkle to this division rivalry.