MAMMATUS clouds are most often associated with the anvil cloud & severe thunderstorms. They often extend from the base of a cumulonimbus but may also be found under altocumulus, altostratus, stratocumulus, & cirrus clouds as well as volcanic ash clouds^
CUMULONIMBUS INCUS at Mykonos, Greece - photo by Sfortis, via Wikipedia; a cumulonimbus cloud which has formed the characteristic flat, anvil-top shape. They can form into supercells resulting in severe storm phenomena such as tornadoes.
The stunning image is of a rainbow cloud, captured above Mt Everest by Oleg Bartunov whilst on an expedition in the Himalayas, Nepal. The amazing rainbow effect is created when tiny ice crystals in the water vapour of the clouds reflect the sunlight.