I've never thought about the life cycle of mimics before, but it makes sense that a creature that imitates a treasure chest might start its life imitating single coins and other small bits of treasure.

Perhaps they birth thousands of young at a time, and what looks like a pile of gold is actually a brood.

DM: “How do you want to do this?” Player: “ I slice the dragon’s cockles with my long and shiny blade” DM: changes song, plays even more intense battle music Player: “What the-“ DM: “Now it’s time to fight the horde! Roll initiative”