What's the trigger for the wild horse pen upgrades?

[Murder of Crows]

I'm only using wild horses this time around, so haven't used the auction (other than the 1st horse).

The auction house upgraded to common horses, but I didn't purchase the upgrade. Is this a trigger for the wild horse pen upgrades? If not, what triggers that?

ETA: I forgot you don't pay for common auction horses. You pay for more horses. Question remains, though: What triggers the wild horse pen upgrades?

[Fyrmare]

Do you have the Meadow Pen, or only some of the other ones? It won't offer the upgrade at all without the "first" pen in the Meadow having been bought.

I don't remember off the top of my head when the upgrade is offered because I didn't get the Meadow pen right away in my last two saves. (One started before the Foal Training and one right after. In the first one I completed the entire story without getting pen upgrades.)

[Murder of Crows]

Thank you! No. I did not have the meadow pen because it has the most normal horses. I finally thought, "Hmmm, I wonder if I need to buy all?" I purchased it & sure enough! Offer for better pens right after.

I should make note of all these triggers & write them down.

[SinisterScarecrow]

Yeah, unless you buy them first, they will not be available.
Which is neat if you only want to catch one type of Wild Pen colouration. Have one of my runs in which I only bought the Rocky Pen to get those pretty Dapples~