Narwhal tusks from the far North Atlantic were collected throughout Renaissance Europe as unicorn horns. Only males have these horns, which are, properly speaking, tusks because they are modified teeth. A few rare males have two.
The Antarctic Ice fish fills its blood stream with an anti-freeze protein to keep its blood from freezing at sub-zero winter temperatures. (Bill Detrich, US Antarctic Program)
The famous clown fish from the movie "Finding Nemo" all start life as males. Only the biggest clownfish in an anemone is a female, and if she disappears, the biggest male turns into a female and takes up her role of egg-layer. (Nick Hobgood, Wikimedia Commons)
All deep sea anglerfish are female. This puzzled marine biologists for a century until they found the answer. What was originally thought to be a worm-like parasite dangling from each female was in fact the tiny male, eyeless, jawless, brainless and gutless. His only role is to fertilize her eggs. (Edith Widder)