There are myriad quantum possibilities, whole universes existing just a molecular vibration away from our own, worlds where things are very different and others where they're almost the same. This is true of the Whoniverse, as newly discovered photographic evidence has shown. I call this echo of our reality "Whoniverse-B", in which the Doctor's adventures are just a tad different...
In Whoniverse-B, it seems the Slyther was a more intelligent breed of monster. Well-read, in fact.
There was also that time the Doctor, Jamie and Victoria fought Yeti with the help of monks in the verdant Welsh hillside. It is not known if the monks were also werewolf worshipers.
Omega's powers over reality-B were much finer, approximating much more humanoid forms for his gellguards, here seen surrounding a civilian.
In his fifth incarnation, the Doctor met young Turlough, an alien going to a public school in the East End.
The 10th Doctor and had adventure on a replica of the Titanic sailing the seas of an alien planet. It almost crashed into a royal palace built into an iceberg.
Don't believe in the existence of Whoniverse-B? I've got further proof, PROOF I tell ya! Just you wait. I'll show you. I'll show you ALL!