An Implant Without a Source

Implant: Hard Skin exists in the game's item data, but no shop, loot table, or reward currently hands it out — and the Pal Surgery Table never asks for it. It's an unused leftover: you can't get it, and you don't need it.

How to Get Hard Skin Anyway

The passive this implant would unlock, Hard Skin (Defense +10%), is one of the default options at every Pal Surgery Table — no implant needed, just 10,000 gold per surgery. Its bigger siblings do need items: Burly Body comes from Implant: Burly Body and Diamond Body from a disposable implant. See the implants guide for how surgery works and which implants are actually obtainable.