Does a creature die outright if they drop below negative max hit points of their polymorphed form? I know a creature will revert to their normal form upon reaching 0 hit points, but a creature dies immediately if it takes damage that reduces it to negative hit points. Does that mean the polymorphed creature dies, or does it just revert and carry damage past 0 over?
No, you transform back before you can be instantly killed
The rule for instant death says:
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
The important part to note is that it only calculates the damage needed to instantly kill you after being reduced to 0 HP.
However, polymorph has a special interaction with dropping to 0 HP:
The transformation lasts for the duration, or until the target drops to 0 hit points or dies.
So when you drop to 0 HP, your form changes. This would happen before the instant death calculation.
The spell then specifically tells us how to deal with damage greater than the HP of the polymorphed form:
If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious.
So, unless the remaining damage brings your original form down to 0 HP and then has enough remaining such that it at least as much as the hit point maximum, you are not instantly killed.
Ruling otherwise is going to be very lethal for PCs and NPCs alike
This ruling makes a lot of sense and ruling otherwise would severely limit the forms that a caster would risk transforming into. For example, most casters would not risk turning into a spider with 1 HP if 2 HP of damage would instantly kill them in that form. The spell is meant to empower the user, not put them at greater risk for death.
But PCs wouldn't be the only ones suffering from a ruling like this. How happy would you be as a DM if your players were able to polymorph enemies into spiders and literally just step on them to instantly kill them?
The creature doesn't die, they revert back to their normal form and take rollover damage. Polymorph says:
The target assumes the hit points of its new form. When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.