Hello Yulyn.
I have a suggestion, but it's just because the dog below is my favorite pet on the site (Some may find it boring, but I love it.) and I think the set would do nicely as a horse adoptable. You could even add a color or two to the list since I think the dog only came in 3 colors if I remember that far back. The foal lines could be like the ones I suggested in the other suggestion thread.
I'd like to see horses patterned after this set of dogs:

It's my favorite pet on the site because it's hot pink and chocolate colored. It has a rival now though in the Halloween chameleon dragon that added orange as a tertiary color. Wow do those colors have me impressed. If you added orange like they did with the chameleon dragon, the resulting horse would be my all time favorite. Thinly or rather moderately (in thin shape) tiger striped legs a bonus but not necessary.
Adding feathered, not demonic wings to one of the above outcomes would make it be my all time favorite, even topping the above dog.

Pegasus please, not avicorn or unipeg or goatpeg or goatcorn. My preference lies in horse bodied pegasi.