Real World article
(written from a Production point of view)

According to the final draft script of the Star Trek: Enterprise episode "Damage", a Droxin field mouse was a type of mouse, one of which lived in Doctor Phlox's menagerie in sickbay of Enterprise NX-01.

In the "Damage" script, the Droxin field mouse in sickbay was mentioned by Phlox upon returning Porthos to Captain Jonathan Archer, Phlox notifying the captain that he had noticed that the mouse had repeatedly been eyed by Porthos, apparently suggesting the dog had an appetite for rodentia.