When you see this mobile-event-place in the city:
you'll know that the Mystery Stables / Secretive Stable master can be purchased
for just 15 BB, at the Supply Store
Once bought, you'll find the animal-seedling in your Animal Tab:
You place it on a 2x2 empty square, and wait 5 hours
After
5 hours, click on your Secretive Stable Master, you will receive
400 EP and your Secretive Stable will change into either any special prize stable or any Classic I/II, XL/XXL pen, Yellow/Orange upgrade
or production pen up to your level (please note that coloured upgrades include the base, so if returned to inventory will split).
Source: Official Farmerama forums.
