Barooga

The Barooga is a kind of non-sapient (but somehow capable of interplanetary travel) insectoid creature.

They need to consume four essential elements to survive: carbon, iron, oxygen and hydrogen; but curiously, they do it in turns. This means that only when the Barooga is saturated with one element will it go search for the next one.

During the "carbon" phase of their feeding cycle they hunt carbon-based life forms; during the "iron" phase they eat metals such as steel; during the "hydrogen and oxygen" phase they just consume enormous amounts of water.