Star Trek: 10 Alien Races That Only Appeared Once
6. Dowd/Trelane
Q might have cornered the market when it comes to the omnipotent, but he wasn't the first and he certainly wasn't the last god-like entity in Star Trek. Through The Original Series (especially) and The Next Generation the audience would encounter super-powered beings aplenty.
The 1966 episode, The Squire of Gothos, cries out for a sequel. With a lively performance from William Campbell as Trelane, he would prove a worthy opponent for Kirk.
The first of two roles Campbell would take in the franchise (the second being Koloth), his powers weren't all he made them out to be. However, that's not dampened the suggestion that Trelane was a Q but only in non-canon materials. Perhaps his easy defeat by Kirk meant he was kept home while the other Q could go out and play across time and space.
In the same vain, the Douwd, Kevin Uxbridge also made a fleeting appearance in The Next Generation. He managed to wipe out not just a shipful of Husnock but their entire race in a fit of rage. In a very Kirk-like move, Picard left him to wallow in his remorse but never went back to check he was OK. Who is to say that Kevin stuck around with the recreation of his wife and their stunning house?