Cyan Beetle

Small, blue, and it lives in the ice caves down at cavern depth, which is about the worst camouflage matchup the player can be handed. Spotting one by eye while you are busy mining is mostly luck. A Hunter Potion outlines it through the stone and a Lifeform Analyzer announces it before it reaches your screen. Between the two the hunt becomes a chore rather than an accident.

Rarity is the other half of the problem. A Water Candle and a Battle Potion push the ice biome spawn rate up, which is the only lever you actually have, at the price of everything else in the biome arriving as well. Clear and light a chamber first so you have somewhere to fight, then let the boosters work instead of walking the whole cave system.

Cyan Dye is the entire payoff, by way of the husk every kill leaves behind. Decide in advance whether you want one dyed piece or a matched set before committing an evening to it.