This article is about Kevin's mutations in the Classic Continuity. You might have been looking for Monster Kevin, Kevin's mutation in the Reboot.
Kevin's Mutations are caused by Kevin's Osmosian ability to absorb DNA and energy. By absorbing DNA and energy from Omnitrix aliens or the Omnitrix or Ultimatrix itself, the energy combined with the DNA will result him to a raw mutation into an amalgamation of some or all unlocked aliens. The Alien Force mutations are not amalgamations of Ben's aliens but rather of different materials (concrete, metal, wood, stone enhanced by magic etc.).