As soon as you put one piece of legendary tack on your horse, it can't be sold anymore. If it is a stallion, coverings offered will be lost. Grand Prix entry will be lost. Morpheus Arms, Nyx Pack items, reproduction and customization items will be lost. A mare in gestation will lose the foal. Once all five legendary tack items are applied to the horse, it will gain a Philosophers Stone.

When the next challenge begins, the story of the legendary horses will be told. The first legendary horse is Altaïr. An author will write the story of the legendary horses, and every time the community has gathered enough horseshoes, a new chapter will be opened. You can fulfill three objectives every day. Objective 1 = two horseshoes, 2= 4 horseshoes and three = 6 horseshoes. Objective three may be solvable with a video. For three objectives solved, the player gains a puzzle piece. You can also go back and solve the objectives you didn't solve the day before.There is legendary tack to be won, and once you have a full set (5, legendary saddle, bridle, saddle cloth, polo wraps and bonnet) you can give them to one of your own horses and trandform it into the legendary horse. The skills the horse has will then double. you'll also get one piece of the legendary star. Legendary horses can not participate in the Grand Prix anymore(!!).

a small promo, water balloon fight where UFOs become water baloon with gifts you can throw at your friends (see an UFO, click it, it reveals a brown saddle, throw it at one of your friends and then both of you will have that brown saddle, for example)
The numbers here are likely to be different, in similar promos, they used to differ from server to server.

Will Altair be able to breed?

When does it come out? Also, just confirming that you can get the 5 pieces of tack from the Golden Fleece? Can someone explain the new GC all over again or point out a page where it is talked about in great detail? Not sure if this needed a spoiler or not but just making sure.