The male seahorse is pregnant, in the full sense

The female deposits her eggs into a pouch on the male's abdomen and swims off. What happens next is not storage. The pouch tissue grows a placenta-like structure that supplies oxygen and nutrients, regulates salinity, and removes waste. The male's immune system suppresses itself so it does not reject the embryos, exactly as a mammalian pregnancy does.

After two to four weeks he goes into labour, with muscular contractions that can last hours, and expels up to two thousand fully formed young. Seahorses are the only animals known where the male is pregnant by every biological definition of the word.