Magma Snail

Bring the right net or do not bother. Swing a plain bug net at one of these and all you get is a burn and a lost snail. You want the Golden Bug Net, or its cheaper cousin the Lavaproof Bug Net if you have not built up to that yet. Ash trees down there will also shake one loose now and then when none are crawling around.

This is the bait that opens up lava fishing, which is the only reason most players go hunting for one. Keep a few in the bait slot and every lava pool in the Underworld becomes a fishing spot worth stopping at. They sell well on their own too, so scooping up each one you pass during a Hellstone run pays for the trip. Crafting one into a Magma Snail Cage is permanent unless you run the cage back through Shimmer, so keep spares if you want both the decoration and the bait.

The shell is close enough to the Demon Conch that the conch is probably just an empty one. Both only turn up around lava, which fits.