For purposes of most polymorph effects, one can divide creatures into types that have basically humanoid shapes and those that do not, as follows:
Humanoid Shapes
Fey
Giant
Humanoid
Monstrous Humanoid
Outsider*
Nonhumanoid Shapes
Aberration
Animal
Dragon
Elemental
Ooze
Vermin
*Most, but not all outsiders have humanoid shapes.
In general, a change from one form that has a humanoid shape to another form that has a humanoid shape leaves all equipment in place and functioning. The subject's equipment changes to match the assumed form. It becomes the appropriate size for the assumed form and it fits the assumed form. The spellcaster can change minor details in your equipment, such as color, surface texture, and decoration.
When a subject changes from a form with a humanoid shape to a form with a nonhumanoid shape (or vice versa) most of his equipment is subsumed into the new form and becomes nonfunctional. Items the subject could conceivably wear in an assumed form remain functional. For example, most items worn on the body, such as armor, cloaks, boots, and most other items of clothing made for a humanoid body won't fit on a nonhumanoid body. Some items can fit on just about any kind of body. For example, a ring fits just about any form that has digits of some kind (the limit of two rings applies no matter how many hands or similar appendages a creature has). Likewise, a necklace fits on just about any form that has a neck.