I believe the page for the objectives is under Achievements in the navigation bar, it's the second one up from the bottom

If you are on mobile you could try:

The special horses page can be found by going into the trophy icon on the far right side of the navigation bar, then scrolling down to "Collections", then into "Special Horses". Pick a random divine family, and click on a divine from it that you don't own. Then hit "Find Out More". That'll take you to the special horses page

I have never tested it as I do not use the mobile version.

thank you very much