The Maze spell states:
Minotaurs are not affected by this spell.
So could an enterprising victim escape the Maze spell by using a spell like polymorph to turn themselves into a Minotaur?
Presumably they can at least make themselves immune to the spell by Polymorphing into a Minotaur before the spell is cast on them, but what about once they've been trapped?
The spell monstrous physique iii allows you to polymorph into a monstrous humanoid, which is minotaur's type. Natural Cunning is listed as one of the abilities you can gain from this spell, if the base form has it, and the minotaur does:
Natural Cunning (Ex) Although minotaurs are not especially intelligent, they possess innate cunning and logical ability. This gives them immunity to maze spells and prevents them from ever becoming lost. Further, they are never caught flat-footed.
Since maze is a spell with a duration, you can't be affected by it if you are a minotaur, since while in minotaur form you cannot be affected by the maze spell or lost in a maze.
According to the rules on immunity, a creature immune to a spell "does not suffer from its effects", which means that even if it began under a spell's effect, it would not suffer the spell's effects once it gained immunity.
A creature with immunities takes no damage from listed sources. Immunities can also apply to afflictions, conditions, spells (based on school, level, or save type), and other effects. A creature that is immune does not suffer from these effects, or any secondary effects that are triggered due to an immune effect.
This specifically refers to the Immunity special quality, but I think it's reasonable to assume that the ability Natural Cunning, which grants "immunity to maze spells", works the same as an ability called Immunity: maze.
Because it's cool, this GM would likely allow a creature that was the subject of the spell maze that afterward assumed minotaur form to just bust out of that maze effect automatically—ideally, bellowing with fury and wielding a battleaxe. Seriously, that's awesomely cinematic, and this GM'd probably let the rules technicalities slide.
However, that said, by default, effects check only if a creature is a valid target when an effect is initially used rather than continuously throughout the effect's duration, and the maze spell says that a minotaur is unaffected by the maze spell. So if a creature wasn't a minotaur before it was affected by the maze spell, becoming a minotaur after it's affected by the maze spell isn't any help against that maze spell. (For assuming minotaur form after being affected by the maze spell to be a valid escape plan, the maze spell whould have to say that a minotaur automatically escapes or dispels the effects of a maze spell or something similar, and that's not what the maze spell says.)
Although not entirely clear according to the rules, this reader has always assumed—and this GM has always ruled—that, absent rules to the contrary, immunity to a spell grants defense against a spell like the spell immunity spell or the extraordinary ability immunity to magic that's possessed by some golems: that is, immunity is unbeatable spell resistance against one or more spells. Thus even were the creature to acquire the extraordinary ability natural cunning of the minotaur, that ability is of no help after the creature's been affected by the maze spell: "This gives them immunity to maze spells and prevents them from ever becoming lost." So, unless fearing another maze spell after escaping the effect of the current one, gaining immunity to the maze spell after being affected by the maze spell is largely a pointless exercise. (Also note that the maze spell doesn't say that a creature is lost within the extradimensional labyrinth!)
In sum, a nonminotaur that's already affected by a maze spell that subsequently employs an effect to become a minotaur just becomes a minotaur affected by a maze effect rather than ending that maze effect.
Based on the discussion in the comments I am going to list a third option:
Being a Minotaur would grant them immunity to the spell and thus they would immediately return back to our world as per the spell description. However:
Each round on its turn, it may attempt a DC 20 Intelligence check to escape the labyrinth as a full-round action. If the subject doesn't escape, the maze disappears after 10 minutes, freeing the subject.
The Minotaur did not perform an intelligence check to escape the maze, their immunity caused the maze to no longer be affecting them, and thus they did not meet the criteria for ending the spell. Think of it much a Periapt of Health and any disease. If they gain immunity to disease it does not cause any disease they are under to instantly end.
As such if the polymorph ends or is dispelled, or if they polymorph into something else before the spell duration is reached, then they will end up back in the maze.
The more humorous problem arises if they end up back in the maze and they had moved any significant distance as a Minotaur:
On escaping or leaving the maze, the subject reappears where it had been when the maze spell was cast.
When the next time they exit, it would be back to the original spot they were standing on when the spell was cast, not where they were when they went back into the spell.
