Aha, so my memory wasn't failing me!
... one per user in March 09, and it's not even on the advent list? Are the advent list pets seriously that rare? xD;
The watermelon dog is at the top of the last draft of the add-on list, and that's from October 08 when so many pets were adopted since there were no adoption limits--I myself adopted out at least four of the watermelon dogs to other users back in 08; they couldn't have been that rare when compared to the 09 st. patrick's day dog, then...? So where does it go?
By the way, just in case these mean anything (rereleases might throw off anything gleaned from this, but I don't think it'd hurt to put this out here):
- roughly 16,000 users joined from Dec 1, 2008 though March 31, 2009 (could adopt advent pets and the st. patrick's day dog if it was available to the end of March)
- roughly 11,000 users joined from Jan 1, 2009 through March 31, 2009 (didn't adopt advent pets but could adopt the st. patrick's day dog if it was available to the end of March)
- the last March 2009 accounts have user ID numbers in the low 29,000's, which means roughly 29,000 people could adopt the st. patrick's day dog IF they were all actively checking CS (which obviously isn't true

)
- and therefore, about 18,000 users could adopt the advent pets (if they were all active, of course)
- by now, a bunch of those accounts are inactive/deleted
I would look at pet IDs as well to see how many pets were adopted in October 08 and how many were adopted in March 09 and then attempt to make guesstimates, but I'm out of time