Sheldon mentioned a few times in "The Big Bang Theory" that Billy Sparks was a bully of his in his youth. Billy Sparks (outside of intentionally scaring him with a chicken in the first episode for his own mild amusement) seems rather cordial with Sheldon in this series when they're together. However, Sheldon is often unreliable when it comes to his past. A great example is how he and Tam drifted apart. Sheldon saw it one way, and Tam took it a different way.