h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There is reputed to be a family of mice that have inhabited the Underground system for so long that they have evolved into a species in their own right. I have on many occasions observed small, brown, mouse-like beasties cavorting beneath the rails at Tottenham Court Road station, so this rumour must at least in part be true.

I also heard a rumour that South Wimbledon station was home to a family of scorpions - any truth in that?

I very definitely saw a badly stuffed dog at Wimbledon station, but he may not count as being strictly speaking a Tube Animal as it is above ground, and possibly on a Main Line platform.

Lastly, I once shared one of those double banquettes on a Northern Line train with a gorilla. He/she moved to another seat as soon as one became available, obviously not wanting to spend another minute sitting next to the Tube Nutter...

Newsflash!! Last Sunday I believe I may have observed one of those commuting pigeons - it got on the tube train at Cockfosters, defied everyone's attempts to chase it off the train at Oakwood and Arnos Grove (both of which are above ground) and then sat tight until we arrived at Finsbury Park, where it simply stepped off the train and waddled off towards the Victoria Line platform...

I knew a genetics researcher at UCL who approached London Underground about 10 years ago to ask to study the Tube mice. The mice were viewed as ideal subjects for gene research because of their unusual 'straight line' habitat, which might be illuminating for studying the spread of traits through populations.
However, London Underground flatly denied the existence of any mice living in the Tube. This amused us greatly because we saw dozens of them every day scampering along the tracks at Tottenham Court Road Tube...

You're right about the Wimbledon scorpions, they do exist!

I think those mice are called "Wombles" or something...