Mad D&D monster idea:
The gelatinous tesseract.
Looks and acts just like a gelatinous cube *except* it extends through the fourth dimension (time). If an adventurer is ingested then fights their way out, roll a d6 to determine which instance they emerge from. (DM needs to pre-plan the history around the monster and work out when each facet exists.)
If you don't kill the jelly you can dive back in and try for another time. But once it's dead you're stranded whenever you killed it.