I played D&D for a while in college, but I didn't love the game enough to really pursue it.
However, I did invent a couple of monsters.
Medium-sized feline creatures who grow chain mail as their pelts. I imagine it as strings of links rather than as a cat-sized jerkin. The cats are valuable but hard to kill, especially in groups. Now that I think about it, they have to be stronger than the default fuzzy feline because they're carrying more weight. The well-defended kittens are especially valuable because their small chain-mail can be used for gloves. This creature is the chain mail lynx.
The other is the mozzarella monster. A wandering blob with a self-generated whip that constricts and burns. There's also a limburger variant with a breath weapon.
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