Baker's Dozen

13 Star Trek/Dr Who one time monsters.

These are the two longest running sci fi series, and both have returning villains, daleks, borg etc. but both have characters that have only appeared once. (This is time sensitive, so as of now, last year Dr Who's Celestial Toymaker and Sutekh could have been on the list, not any more.)

1 The Dominators/quarks from Patrick Troughton's Dr Who episode of the same name.
2. Armus, tar monster that killed Lt. Yar from Star Trek: TNG
3. The Voord from The Keys of Marinus 1st Doctor.

13 Star Trek/Dr Who one time monsters.

These are the two longest running sci fi series, and both have returning villains, daleks, borg etc. but both have characters that have only appeared once. (This is time sensitive, so as of now, last year Dr Who's Celestial Toymaker and Sutekh could have been on the list, not any more.)

1 The Dominators/quarks from Patrick Troughton's Dr Who episode of the same name.
2. Armus, tar monster that killed Lt. Yar from Star Trek: TNG
3. The Voord from The Keys of Marinus 1st Doctor.
4. The Providers, Star Trek: TOS

13 Star Trek/Dr Who one time monsters.

These are the two longest running sci fi series, and both have returning villains, daleks, borg etc. but both have characters that have only appeared once. (This is time sensitive, so as of now, last year Dr Who's Celestial Toymaker and Sutekh could have been on the list, not any more.)

1 The Dominators/quarks from Patrick Troughton's Dr Who episode of the same name.
2. Armus, tar monster that killed Lt. Yar from Star Trek: TNG
3. The Voord from The Keys of Marinus 1st Doctor.
4. The Providers, Star Trek: TOS
5. Axos From Claws of Axos 3rd Doctor.