You first have to go to "manage estate" and create tabs for your horses. Then go to your horse's page and then to the edit section. From there you can select a tab for your horse. I hope this helps 

To add onto this; you can also tick the boxes to mass move horses to created tabs and select the tab from the drop down menu, at a smoother transition all under Create a Tab section. It also helps to number your tabs in order so if you need to freeze tab(s), it's easier to do so once numbered.

Ohh! Ok thank you!