The exact answer is unknown. Shukaku's jinchuriki don't seem to have access to any of the other tailed beasts' jinchuriki transformation, instead covering their bodies with sand in order to create a replica of Shukaku. Since Shukaku controls sand, he encases his Jinchuriki in sand to increase defense and offense, but it has the same effect as Killer B and Naruto's cloaks. Shukaku's Tailed Beast Mode is also made of sand, it is something special to Shukaku, as all of the other Tailed Beasts manifest their body through chakra.