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The Cash Shop was introduced as a way to help pay to keep PokePlushies running, and can be found here. Each adoptable costs $3.00 USD and can be easily paid for through PayPal. For those of you who can't buy these for whatever reason, my personal suggestion is to buy a prepaid Visa from Walgreens, or similar drug store. These can be used on PayPal, and have a set limit built-in.

Swiftwings
Swiftwing Red & Black Swiftwing Blue & Orange Swiftwing
Swiftwings were among the first Dinomon adoptables available in the Cash Shop, and were also the first to be retired.

Wuris
Volcanic Wuri Flower Wuri Mint Wuri
Wuris were available for purchase at the opening of the Cash Shop, and unfortunately, cannot be bought any more.

Light Cherions
Air Light Cherion Earth Light Cherion Fire Light Cherion Water Light Cherion
Drigons and Light Cherions were introduced to the Cash Shop at the same time, and are also the first sets of elemental adoptables.

Dark Cherions
Air Dark Cherion Earth Dark Cherion Fire Dark Cherion Water Dark Cherion
Dark Cherions are the evil counterparts of the Light Cherion. They are also the third elemental set added to the Cash Shop, Like most new adoptables, the first Dark Cherion was discovered on the Click Exchange.

Drigons
Air Drigon Earth Drigon Fire Drigon Water Drigon
Drigons and Light Cherions were introduced to the Cash Shop at the same time, and are also the first sets of elemental adoptables.

Dragon Cylins
Spring Dragon Cylin Summer Dragon Cylin Autumn Dragon Cylin Winter Dragon Cylin
These four were the first Cylin available to acquire in a way other than winning a contest. They are also the first set of adoptables with a seasonal theme, and have since been retired.

Kayas
Kaya of Love Kaya of Joy Kaya of Life Kaya of Harmony Kaya of Wisdom
Kayas are special new creatures, listed in the official adoptable guide under Cylin Nights. Although it is a bit tricky to see, there is a symbol on the scarves Kayas carry. Each Kaya has a different symbol.

Zippy Ornapet
Zippy Ornapet was released for the 2008 winter season. He is also the very first animated adoptable. Zippy is set to hatch on Christmas day into something no one knows yet.

Others
Theodore's Speech Bubble is the only adoptable you can earn three different ways-- Cash Shop, Dinomon Handlers, and in Cyde's Maze. Unfortunately, this adoptable is now only available in the Cash Shop.
Cyde's Plant was added to the Cash Shop as a prank, and was first priced at $100.00 USD. After many complaints, the price was raised to $1,001.00 USD, but the adoptable was later really released for finding a secret password. This password no longer works to get you an adoptable.
Meet the Kuron. This is a special adoptable given to the people who buy from the Cash Shop. Every adoptable you purchase gives you one piece of Kuron food, which you can feed to your adoptable to gain levels. The egg hatches at level 25.