This kit is the SD Soutenmaru & Koutenmaru, released as part of the SD Gundam Musha Maruden 3 series. As the name suggests, it's a two-kit combo pack — the blue Soutenmaru and the red Koutenmaru can each be reconfigured into Souten and Kouten respectively by removing or repositioning their armor.
The weapon and accessory lineup is a bit involved. You get two small swords, Tenzam and Netsuzam, which can be combined into a single weapon called Stainless. The Sakuikanlaikki — which even uses wire parts — can have additional parts attached to become the Fujoushinkou, and removing the handle from the Nettouennogu turns it into the Ceramic. You can then combine and mount the Fujoushinkou, Ceramic, and Tenzam/Netsuzam together — though honestly, with all these unfamiliar names it's easy to lose track of what's what, so I'd recommend just looking at the photos to get the idea.
The real highlight of this kit is the combination feature between Soutenmaru and Koutenmaru. Transform Soutenmaru into a horse-like form and have Koutenmaru ride on top, and you get the four-legged Ounetsu Kirin. Add the remaining weapons and armor on top of that, and you get the final combined form, the Nokirin.
So yeah, it's a unique kit that gives you two mechs in one box and lets you combine them into a horse-like form. For 800 yen you're essentially getting two kits' worth of content — pretty great value for an SD Musha Gundam. :-)
AIThis page was translated by Claude Sonnet.