Acid-breathing sharks are not only non-preposterous, but likely essential for any TRUE Dark Overlord or Black Dragon! Given enough time, and with a natural immunity to acid, why not make giant pits of acid for enemies to fall into? It's a great death trap.
But wait...
Oh yeah, there's spells called Energy Resistance Acid and Energy Immunity Acid. Well pooh, how WILL you keep the giant acid pits a credible threat? Well obviously the answer is to fill it with hostile and hungry creatures and whats more hostile and hungry then an acid-breathing shark?So there.
I for one embrace the acid breathing shark and it's lava breathing cousin as true additions to an evil overlord's arsenal of villainy.