Shiromari

is a chameleon summon that takes the form of a castle and the personal summon of Kōza Kubisaki.

Background
It was summoned over fifty years ago. As a last resort, the dying lord of the castle summoned Shiromari and had it devour anyone who approached. The lord died before he could release the summoning, and thus it remained bound to the contract scroll used to summon it.

Cursed Castle Arc
During a mission to find a group of missing people, Naruto, Hinata, and Kiba stumble upon this summon.

After being swallowed by it, they realise just what it is. While attempting to find the contract scroll that is keeping it summoned, the spirit of the castle lord appeared and led them to it, because the lord felt sorry for the creature spending fifty years guarding his castle, and wanted to set it free. The summoned creature is very loyal to his summoner, having protected his summoner's castle for so long. Naruto cut the scroll into pieces to send the summoned creature back, and escaped from the body of the summon with Rasengan. The anime does not show Shiromari's true form, but instead, a picture of what appears to be a chameleon.