UPDATE 6th October 2015: Metal Gear Solid 5's costume DLC is live now. [Editor's note: It appears that this is only on Xbox One and Xbox 360. The PlayStation and PC versions have yet to arrive, but an alleged screencap of a since-removed listing on the PlayStation Store suggests a 20th October release.]
Each DLC item, including the horse armour, costs 79p.
Here's the official description of the Tuxedo skin:
Tuxedo that Naked Snake is said to have worn during Operation Snake Eater. For the gentleman who knows how to dress for the occasion.
And here's the description for the Western Tack horse armour:
Non-tactical loadout for D-Horse. Perfect for imaginary travels through the Old West.
I quite like the description for the Parade Tack horse armour, too:
Non-tactical loadout for D-Horse. Special tack for ceremonies, parades, and other formal occasions, real or imagined.
This one, though, for the Jumpsuit, is a tad curious:
Jumpsuit that the female spy EVA wore during Operation Snake Eater. The chest area can be unzipped for a tactical advantage. *For female staff only.
There's also a Sneaking Suit for female staff only.
White Sneaking Suit that The Boss wore during Operation Snake Eater. In addition to the noise-dampening soles that eliminate the sound of footsteps, this suit features a high-strength aramid weave that offers superb damage resistance. *For female staff only. *In FOB missions, this uniform prevents your body heat being detected by NVG-equipped enemies.
Here's Fatigues for Naked Snake:
Standard-issue combat fatigues worn by Naked Snake during Operation Snake Eater. Wear on sandy surfaces or among greenery to blend in, making it harder for the enemy to spot you.
And finally, the Sneaking Suit for Naked Snake:
Soviet Sneaking Suit worn by Naked Snake during Operation Snake Eater. In addition to the noise-dampening soles that eliminate the sound of footsteps, this suit features a high-strength aramid weave that offers superb damage resistance. *In FOB missions, this uniform prevents your body heat being detected by NVG-equipped enemies.
ORIGINAL STORY 17th September 2015: Metal Gear Solid 5: The Phantom Pain will be getting costume DLC.
Revealed during Konami's Tokyo Games Show panel (via JunkerHQ on Twitter), the DLC will consist of a couple pieces of ornate horse armour, along with a handful of wearable costumes from Metal Gear Solid 3.
The men's outfits consist of a tuxedo, MGS3's version of stealth camo, and a new set of jungle fatigues. The women's wear (which can be equipped by any female prisoner you've rescued, who you can then play as) includes the trademark outfits of both The Boss and Eva from Snake Eater.
No mention was made as to the price of this DLC or when it's coming, nor did Konami say if any story DLC is in development. Given Kojima's spotty relationship with the publisher, I wouldn't hold my breath on that.
Here's pics of the upcoming outfits: