Yes, hydra is a Scrabble word!

Noun

1. (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones

2. A long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer

3. Trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality

4. Small tubular solitary freshwater hydrozoan polyp