Mystery Theme #LP16 by PolinskyL

Question 11

(For the sake of keeping the answers in order, this question is here and not at #12. Feel free to answer #12 then come back here.) Until about a year ago, the answer to this question would have completed the set. Please identify either the mystery theme or the one member of the set not referenced in any of the other answers.

Affirmed / Triple-Crown winning horses