For Pathfinder?

It's a Continuous item of the appropriate X-Shape spell, with a limitation that it's only doing one specific form. So an item that turned the wearer into a Medium Red Dragon would be based on Form of the Dragon I (and you could use the breath weapon... maybe 1/day, or once every 11 minutes?) and would give the user "a +4 size bonus to Strength, a +2 size bonus to Constitution, a +4 natural armor bonus, fly 60 feet (poor), darkvision 60 feet, a breath weapon, and resistance to one element. You also gain one bite (1d8), two claws (1d6), and two wing attacks (1d4)", "All breath weapons deal 6d8 points of damage and allow a Reflex save for half damage", and "30-foot cone of fire, resist fire 30, vulnerability to cold"

It's a 6th level spell, so Caster level 11. Minutes/level. Formula for a continuous item is thus:
Spell level x caster level x 2,000 gp * 2 (Minutes/level) / 2 (Red Dragon only) = 6 * 11 * 2,000 gp * 2 / 2 = 132,000 gp. A little refluffing also lets you have it grant a variation on the half-dragon template.

On the other hand, if it always turns someone into a human, it's instead based on Alter Self, with the same formula, just different numbers. Spell level 2 * caster level 3 * 2,000 gp continuous base * 2 for minutes/level /2 for specific form only = 12,000 gp, and it grants a land speed of 30 and a +2 Size bonus to strength.

If you're the DM, just grab the appropriate Form or Shape spell for what you want to apply, snatch the actual mechanics from there, price it out, and refluff as needed. A curse making it hard to remove is optional, and probably should not affect the market price much.