Any small fresh-water hydroid of the genus Hydra, usually found attached to sticks, stones, etc., by a basal sucker.

a freshwater hydrozoan resembling a tiny sea anemone, an aquarium pest that is sometimes a problem in breeding because it preys on fry.

small tubular solitary freshwater hydrozoan polyp

a small animal that lives in the pond

Hydra is a genus of simple, fresh-water animals possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most un-polluted freshwater ponds, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas.