Bikyaku Mimic, Harumi-san: Tensei Monster Isekai Nariagari Densetsu

Harumi can't remember much, like who she is or where she came from, but one thing's for certain... She's somehow been reborn as a treasure-chest-shaped monster—a mimic! And no sooner than she gets her bearings, she gets unceremoniously dumped into a labyrinth full of other monsters and told she must survive for at least five days. Between treasure-hungry adventurers murdering their way through the labyrinth and monstrous rivals joining the fray, that won't be an easy task for a newborn mimic! But although it seems like Harumi's received the short end of the reincarnation stick, she has an invaluable asset on her side. They say that Helen of Troy's face launched a thousand ships, and, well...it looks like Harumi's legs can do the same!

(Source: J-Novel Club)