Greetings, welcome to the Forums.
How fun. You seem to have got a second 'free' plant. Likely a seed or piece of rhizome somehow found its way into the potting soil.
How long have you had the Begonia?
The mystery plant appears to be some type of Aroid (Araceae), possibly an Alocasia, Colocasia, Syngonium, etc. I'm not sure.
I would re-pot the volunteer into its own container and watch it grow. Once it is larger, the leaves should be identifiable.