Someone's pointed out to me that making a bid on a beast that's up for auction checks that your stables are not full, but doesn't seem to pre-allocated a space in your stables (the way a pregnant beast does) to make sure you have space in your stables when the auction ends.

In my quick search of the FAQs and forums, I can't find anyone mentioning what happens if the highest bidder doesn't have stable space at the time the auction ends. In that situation, what happens to the beast? Does it still go to the highest bidder, even if they'd then have more beasts than their stable capacity allows?

I have run the experiment.

If you win an auction but don't have free space in your stables, you'll still receive the beast from the auction. Your number of beasts (displayed at the top of the pets page) can go above the pet limit (eg. 51/50). (But, of course, you can't get any more beasts while you don't have free space.)