When I gain control of a creature, do I also gain control of any auras or equipment attached to it?
Your opponent controls the Nighthowler.

Control of an enchantment and control of the object it enchants are independent.

Changing control of a permanent doesn't automatically change control of any Auras or Equipment attached to that permanent. Your opponent still controls the Nighthowler, even while you control the creature it enchants. And your opponent will retain control of the Nighthowler if it becomes detached from the creature you've stolen for any reason (e.g. that creature leaves the battlefield or Nighthowler can't enchant it anymore because it gains protection1), because none of the game actions described are changing control of the Nighthowler.



1 - The rules for bestow+protection are a bit poorly worded, but Rules Manager Matt Tabak has confirmed that this is the correct interpretation.