Hemopiranha

Rage Potions come from these, and rage covers the critical strike half of the pair that Wrath completes. Running both at once is standard for any serious boss attempt, so a crimson world wants a fishing pond dug early rather than at the moment you need a pile of potions overnight.

They bite at any depth inside the biome, so there is no reason to go down to the caverns for them. A pond cut into crimson grass on the surface is enough and far safer than fishing next to a chasm. The same fish feeds the top tier cooked meal, which is worth remembering if you would rather eat than drink.