The Bloodhound was a breed of domesticated dog, originating from Belgium, on Earth. The breed was known for being good at tracking Humans and other lifeforms, and as such, individuals who possessed similar abilities could be referred to as bloodhounds.
In 2152, Trip Tucker described himself as "a regular bloodhound" when Jonathan Archer needed to make a return trip to a pre-warp civilization to recover a missing communicator. (ENT: "The Communicator")
In 2153, Malcolm Reed likened the Xindi-Reptilian seekers to Bloodhounds. After destroying one seeker and damaging a second seeker on a Xindi-Arboreal colony, Reed hoped that they "stopped those Bloodhounds before they transmitted our position." (ENT: "The Shipment")
When the crew of the USS Voyager was allowed to experience the Q Continuum firsthand, they found an old roadhouse along a desert road. A tired bloodhound was part of the representation. Quinn mentioned he had traveled the road many times, sat on the porch, played the games and had even been the dog. (VOY: "Death Wish")