The creature that doesn't untap during its controller's untap step is the one that Heavy Arbalest is equipped to as that untap step begins. If a creature was tapped to activate the ability granted to it by the Arbalest, but the Arbalest is no longer equipping it, it will untap as normal.

The equipped creature is the source of both the damage ability and the resultant damage, not Heavy Arbalest. For example, if Heavy Arbalest equips a green creature, the ability can target a creature with protection from artifacts but not one with protection from green.