top one is ( i think) a new update coming along with the new great challenge, bottom one is apparently the divines information

How many are there?

There are going to be 5 Legendary Horses in total. While the full details are only known on one of them at this time - we do have pictures of the other four:
Altair
Bucephalus
Amira
Zaldia
Tormenta

How do they work?

Legendary horses are very similar to Divines in how they work. They can not breed*, can not be sold, offer a perk to their owner, do not count for EC Prestige, and can only enter One regular competition a day - but unlimited Divine competitions.

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How do I get one?

You need to collect the 5 pieces of Legendary tack and give the tack to a Normal horse. Once the horse has all 5 pieces - it will transform into Altair. However - keep the following in mind when choosing a horse:

1. The horse you give a Legendary tack item to will no longer be able to be sold

2. Once you give a horse all 5 pieces - it will 'transform' into Altair - just keep in mind that:

a. The horse will lose any items it has on it - but gain/keep a Philosopher's Stone *all other items are lost and can not be recovered*
b. A stallion with any unnacepted covering offers - will have those coverings cancelled
c. If you are using a Mare who is pregnant - she WILL lose the foal
d. Your horse's skills will double
e. The horse's birthdate will change to the date of transformation
f. The breeder's name will change to your name
g. The affix the horse has will be lost*

All of this is similar to how the transformation of Junior Croesus works (ex. the Junior Croesus is deleted and you are given a Croesus in its place...Except in this case, the skills will reflect that of the horse you transformed - but be doubled...but all other information about the horse will be lost))
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How can I get the tack?

You can get the tack by:

Winning it from Great Challenge objectives
Once the festivities start (or before hand if you are a VIP) - by finding it in the Golden Fleece
In the Lottery once the festivities start*
By purchasing a 'tack' Pack *Cost unknown at this time*
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You keep mentioning Altair and said there's more info on him - who is he?

I don't like Altair's tack/specialty/background/coat - can I change it?

Far as I know - you can not change the tack or specialty of these horses. As for the background/coat - Legendaries seem to be the same as Wilds in that the background they have is set (and their coat can not be changed like any other Divine/Wild horse)

In a few other spoilers about these Legendary Horses - I've seen this thing called the Star of Immortal Power, what's that?

Star of the immortal power

*this is a shot of the Legendary Horses page that will show in the directories once they go live

When a horse becomes a legendary horse, you receive the branch of a star. Each branch can only be obtained once. For example, if you turn 2 horses Altaïr, you will receive only one branch of the star.

When you have collected the 5 branches of the stars (and therefore get the 5 legendary horses), you can activate the star.

Upon activating the star, all horses owned by the player receive the Star of immortal power bonus, which gives them +10 in each skill. The star can only be activated once.
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So according to the Star of Immortal Power - I can get more than one Altair?

Yes, provided you can collect the 5 pieces of Legendary tack again, you can get as many as you want (you just won't get additional pieces of the star).

*unconfirmed information as no one but the Admins have the Divine at present time

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
3= 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(!!).

How do you get the tack?

The tack in the lottery but you wouldn't get all of the tack just there

These horses are very special 5 in number and each will be in line with a story. The first story will be revealed chapter by chapter with the progress of the collection of horseshoes Grand Challenge. The other stories will be revealed by other competitions in the future.

Altair is the first of five legendary horses. You can give pieces of equipment Altair to one of your horses. When you have five pieces of equipment, your horse becomes Altaïr. It keeps the same features but these skills doubles. The horse loses transformed these objects of Black Market: Arms of Morpheus, Nyx Pack, reproduction objects, customizing objects. Registration for the Grand Prix is ​​lost but the competitions are underway. During protruding offers are lost. If the converted horse is a mare in gestation, it will lose the foal. So pay attention to the horse you turn! The horse turned will receive the Philosopher's Stone.
PS: it is impossible to put the equipment on a horse with a Poseidon's pack

The duration of the festivities depends on the number of days that the community needed to achieve the goal. So: the faster you collect the irons, longer the Festivities will be.There are 5 pieces of tack. When you have them all, the horse on which you have chosen to place them on will turn into Altair.

I really want to get the last 2 legendry horses because they look so amazing