Definitions of hydra:

• noun:   small tubular solitary freshwater hydrozoan polyp
• noun:   trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality

  Example: "We may be facing a hydra that defies any easy solution"

• noun:   a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer
• noun:   (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones

  Example: "Hydra was slain by Hercules"