Need Backstory Inspiration: Attacked by what Monster?

So I'm about to start a new campaign and I'm looking for a last missing piece in my backstory.

So long story short, I'm building a rock gnome artificer. He grew up in a shop, but he knows lots of stories about his grandfather, who was an adventurer/archeologist type. He wants to be like him.

In one of his first cave dives, he loses his leg to a monster. And I don't know which monster would be suited for that. The obvious choice would be to use a mimic, but since it is that stereotypical I would prefer something else. While the char is smart (high INT) he is quite naive and not too perceptive (low WIS) in regards to something sneaking up on him.
Any ideas on what kind of monster would be suited for this? The character was alone and is saved by a cleric NPC only a while later, so they won't slay the monster, only find my body.

Also, losing the leg doesn't change much about the character dynamics, since he's an battle smith artificer, so crafting a prosthetic leg for himself is not a problem.

Thanks a lot in advance.