Some items can be transformed to other items, like the Cat Grooming Kit that can be turned into the Dog Grooming Kit and also back. None of the items are rarer than the others because you can choose what you want from the Collection.
Some of the items (I believe the snake and the cat/dog) are transformable. They're one item, but can transform into the others.
And you pick and choose what you want now. There's no 'rarity' involved.

Some are transformable - like the wigs and now animals (the cat grooming kit turns into the dog one) typically are. You can also only pick one item per bag.
