No, you can't get a Dev Creature just by playing the game, they're only obtainable through Robux purchase or by trading with another player who already owns one.

The difference between a Palette and a Plushie is that a Palette is a color variant applied to a creature you already own, while a Plushie is a separate cosmetic collectible on its own.

No, item values in Creatures of Sonaria don't stay fixed, they're community-tracked and shift based on demand and supply, similar to how Roblox Limiteds work.

Yes, you keep the Creatures, Palettes, and Materials included with my purchase, exactly as shown in the listing.