The Frog Merchant is found commonly underground, when you find him, there will be grey bricks and frog statues that lead to the Frog Merchant.
The Frog Merchant wears a skull and cape with a long purple coat, and he has a bag behind him. Oddly enough he, and his bag appears to be an interractable object, rather than an NPC, as when the blocks under him are destroyed, he remains floating, and the background blocks behind him and him and his bag remain untouchable. He remains invincible to any sort of attack, be it bombs, weapons, or pickaxes, nor do monsters affect him, or even take notice to him. He can be found with dogs running around him.
He is selling: