Check out the Hydra Region page which is part of the Texas Astronomical Society's Constellation of the Month Series.
Hydra, also called the serpent of Lerna, was a beast with the body of a hound and 100 serpentine heads. As if this weren't bad enough, it also had poisonous breath and it was so hideous that it caused most people to die of fear from simply seeing it. One of Hercules' great tasks was to kill this monster. When he started to fight it, he discovered that every time he cut off one of the heads, three grew back in place of it. Seeing this was getting him nowhere, he had his charioteer, Iolus, burn the stump after each time he cut off a head, which prevented the unfortunate regeneration. The last head was immortal, however, so after cutting it off, they trapped it under a rock.
Hydra is a very weird constellation. It is the largest constellation on the sky (with Virgo close behind), but is strung out from Libra to Canis Minor along the southern horizon over about 90 degrees. Believe it or not, it used to be even bigger, but like the old Argo Navis constellation, it was split into four parts. Various stellar cartographers, including Flamsteed and Hevelius, broke the old Hydra into Sextans (the sextant), Crater (the cup), Corvus (the crow) and a new, reduced Hydra.
Felis, the Cat In addition to these three new constellations, an obscure French astronomer added a constellation of his own to a star chart he created. In between Hydra, Antlia and Pyxis, he drew a little cat and named it Felis. However, Felis was not officially adopted, so it remains merely an anecdote...