Gelatinous Cube

This is a famous monster from role playing games. One creature is just a unique blob with unfinished business and the other has remnants from previous encounters.
Imagine you are entering the dungeon and a slow moving creature advances. It catches one of your adventurers by surprise and is instantly engulfed in the creature. Another creature of the same design slowly closes the path behind you. They seem to have been hidden from your view initially, but now they are deadly.