For this, I'm going to separate the different beasts by body type: Biped, Quadruped (hoofed), Quadruped (clawed), Avian, Arthropod, Piscine, and Serpentine.
Which of these body types would you allow to wear the following magic item types?
Amulets/Brooches/Necklaces/Periapts?
-Amulets and necklaces inherently require a neck, that is to say a narrow point between two wider points. So anything creature that fits that description could wear those. Brooches are typically worn on clothes, so you'd have to be able to wear non-magical clothes. The best definitions I come up with for periapts say they're also necklaces. So, same deal.
Armor?
-Resize within a range. Bipedal armor fits bipeds. Quadrupedal armor fits quadrupeds, specialty armor for specialty creatures fits specialty creatures.
Belts?
-As a belt is really nothing more than a circle around a midpoint on a creature, so long as that creature
has a midpoint, it could wear them.
Boots/Shoes?
Same as with armor, they can resize/reshape within a range.
Cloaks/Mantles/Robes?
Being little more than a blanket with a hood, as long the proportions of the creature weren't vastly out of expectations, cloaks and mantles would IMO be one-size fits all. Robes are clothes designed for a specific racial shape. I'd give them more leeway with the specifics of the form, but generally limited as armor.
Gauntlets/Gloves?
Must have hands. I'd assume a magical gauntlet could adjust a finger or two if need be, but the magic has to fundamentally understand that what it's being put on as a "hand". So if it doesn't have hands, no go.
Glasses/Goggles/Lenses?
Must have the number of eyes the item was designed for.
Hats/Helms?
Hats, everyone gets hats! But helms are designed for a specific head shape, so again, adjust within a range.
Rings?
Provided it has a body part that is roughly cylindrical in shape, I don't see why a ring couldn't be put on it. And
all that that implies.
Suffice to say, everything in my world that does magically reshape does so within a range. Armor designed for dwarves still inherently has two arms, two legs a single torso and a single head slot, so becoming taller and thinner to fit an elf is a reasonable range. An armor designed for someone with 4 arms does not suddenly lose the extra two arms when a two-armed creature wears it. Armor designed for a cat does not work on a human, but may adjust for a dog or a horse.