The swordtail is a type of fish that is popular in aquaria. They originate from riversin South America, and reach a length of only 3 inches at most.
The feature that gives the swordtail its name is the long protrusion from its tail. This only occurs in male fish, and is not hard. There may be several of these, not just one. It is this feature, as well as the vibrant colouring, which may be that likely account for its popularity.
The most notable courtship behaviour is that of the male swimming backwards when trying to attract a female. During mating the male lies underneath the female. The swordtail is a live bearing fish, like mammals the egg hatches inside the mother, and develops to a certain point before birth. Pregnant females (recognisable by their swollen appearance) should be kept in a different tank from the rest of the fish, and then replaced as soon as they have given birth. This is becauese they will try to eat their new young. This possibly evolved as a mechanism to prevent overcrowding. Once the fry have grown enough they can be moved to the main tank.
One source I've seen, that claimed they live in small ponds gave me information for the material below, but Wuukiee
The "sword"'s main purpose is to help the swordtail move across areas of mud. In their natural habitat the ponds that swordtails live in often dry up, and to survive they are forced to move to another expanse of water. Unlike some other fish they are not able to survive out of the water for a long period of time.
Disclaimer: don't buy any fish without doing the necessary research first. This is not intended as a guide to swordtail owners, only the general public.