Originally, in the Zodiac developed by ancient Babylon, there were only eleven signs. Scorpio took up a double-sized slice of the sky and was immediately followed by Virgo.
Libra is a relatively young sign.
The mythology relating to Virgo is very difficult to pin down because there are so many maidens spread across mythology. The most common, though, have been Ishtar, Demeter, or Astraea.
At some point, Scorpio's claws were cut from his constellation and attached to Virgo as her scales. This made Virgo the blindfolded goddess of Justice we know today. But that still didn't work, because now Virgo took up an oversized slice of the sky.
The ancient Romans solved the problem by cutting the scales from both Scorpio and Virgo and making them a constellation--and a Sign of the Zodiac--in their own right. Libra remains the only sign that is not a person or an animal.
The sign of Pisces, a faint constellation, has always been known as two fish. The Greek myth most likely to be associated with Pisces is the story of how Aphrodite and her son Heros escaped from the monster Typhon. The pair turned into fish and tied their tails together to make sure they weren't separated. Even today, many depictions of Pisces show a line or bar connecting the two fish.
One of the oldest signs of the Zodiac, Aquarius is widely associated with the Greek myth of Ganymede, the cupbearer of the gods, and the first mortal ever to be turned into an immortal. Zeus had a fondness for him, turned into a bird, and swooped down to carry him off to Mount Olympus as their servant, a sort of mythological cabana boy. Ganymede is surrounded by water constellations. Near him are Pisces the fish, Eridanus the river, Delphinus the dolphin, and Cetus the sea monster.