Can you target your own creature with a spell that says 'target creature an opponent controls' if an opponent gains control of it?

The phrasing 'target creature an opponent controls' restricts targeting to creatures under an opponent's control at the time you choose targets. Ownership and control are two different concepts in Magic. You own a card if it started the game in your deck, but you control a permanent if it is currently on your side of the battlefield under your authority (CR 108.3, 109.4).

If an opponent has gained control of your creature — through effects like Threaten, Act of Treason, or Mind Control — that creature is now 'a creature an opponent controls' from your perspective, even though you still own it. You may legally target it with a spell that says 'target creature an opponent controls' (CR 114.1, 114.9).

Conversely, if you control a creature that started in an opponent's deck (i.e., you took control of their creature), that creature is now 'a creature you control,' so an opponent could target it with such a spell, but you could not.

Concrete example: Your opponent plays Act of Treason, taking control of your Grizzly Bears until end of turn. Before end of turn, you cast Pacifism, which reads 'enchant creature an opponent controls.' Grizzly Bears is now controlled by your opponent, so it is a legal target for your Pacifism — even though you own the bear.

The key rule is CR 109.4: 'A spell or ability that requires a target will check the legality of the target at the time that spell or ability resolves.' But legality for targeting purposes is first checked at the time of target selection (CR 601.2c), and control is evaluated at that moment based on who the permanent's controller is, not who owns it.