Kangaroo words contain another smaller word that is a synonym, as roo is a smaller (baby) kangaroo, e.g. illuminated contains lit.

Some kangaroo words do fall into Hydra's definition, cleanse for example, though many do not as it doesn't require the letters be adjacent.

A special case of Hydra's kind in which the whole word can be reanalyzed:
therapist = the rapist