Cards are one of my personally favourites. While the price was increased for buying cards it is in my opinion one of the easier promos to stock items on.

Tormenta is also a Mare. Zaldia's gender is unknown at this time. However, note that even though some of the Legendary Horses are Mares - they can NOT reproduce.

Diamonds would be the only other way to get Cards outside of using passes, and the ways the game offers them (Logging in, winning comps, and the Flash Sales).

Though if you are super lucky - it is actually possible to get the Legendary from the cards without needing to use passes (though you need to be lucky to always get puzzle cards, and get the ones you are missing - and get the prize cards for her tack)

And the VA promotion is very easy - it takes no more than 10 minutes to do...so highly do-able on a school/work day. It's more or less a 'scavenger hunt' in visiting different pages.

The Vintage Apple promo is on the eighth of September? Will it start right after the update and go until the next one?

what is the next thing after this and where is the clues on the treasure hunt

While it is confirmed that the event will start on the 8th and run until the next update, the start time is unknown (so it might start shortly after the update, it might not start until a little later in the day). We'll just have to wait and see.

After the Treasure Hunt ends, it will be the COntinents Cards. While the exact start date is unknown, my guess is sometime next week.

The Vintage Apple (Owlient's 10th Birthday Celebration) event will also be happening on September 8th and running for one day only.
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As for Treasure Hunt Clues - they only show up once you hit a target amount of points. These are the rough point values people have found the Clues at:

Scene 1 = 41,000 *Final prize at 42,000 points*
Scene 2 = 50,000 *Final prize at 54,000 points*
Scene 3 = 60,000 *Final prize at 66,000 points*
Scene 4 = 66,000 *Final prize at 78,000 points*
Scene 5 = 75,000 *Final prize at 90,000 points*

As for where the Clues are - some of these images are out of date, but they will help you see where they are and what they look like:

Scene 1


Scene 2


Scene 3


Scene 4


Scene 5

Yay! As for the card event, I was under the assumption that you'd have to use passes to buy cards, and they weren't available from any other methods. Thanks for clearing this up!
What, exactly, are flash sales, though? Are they simply a sale on cards? Sorry if it's obvious, I haven't been through very many events.

Sept 8, no info on if it's from update to the next update, or just starts on a certain time of day.

The Hoof Prints in the sand, to the bottom left, I believe.