Johannes Merrimack III or Mr. Merrimack was a Banker who worked at the Bank of Dirt. He was apparently a family friend of Beans, consider her a niece, and knew her father.
He seemed to had caught on to the Mayor's schemes and refused to help him. For this, he was murdered and his body left in the middle of the desert, where it was found by Rango's posse. Doc determined that Merrimack died of drowning.