ADQ: We encountered a human who
killed all the characters by breathing
fire on them. The DM said it was a
Polymorphed red dragon (one of its 4th
level spells). Is this allowed?
ADA: Your DM can do whatever he or
she wants, so it's allowed -- but it's not
by the rules, and it doesn't seem fair.
The ADVANCED D&D
game rules would allow a Polymorphed
dragon to move as a human,
use human-sized things, and so forth,
but to use its breath weapon, it would
have to resume its dragon form.
(Polyhedron #12)


 

T1-4.126 notes that sparkling lights usually accompany a polymorph.

--> Main entry for questions is at *polymorph self*.
--> MP: 24.