Gossamer mouse

A gossamer mouse was a semi-transparent mouse-like mammal known to the Federation in the 23rd century.

Gossamer mice had a furry body with large round ears and were about four inches long with a narrow tail. They were highly sensitive to environmental changes.

History
In 2269, a pair of gossamer mice were kept with a group of experimental animals in the medical section aboard the USS Enterprise (NCC-1701). Following exposure to spiroid epsilon rays, Leonard McCoy reported that the gossamer mice had not reacted, suggesting that the rays had not harmed the crew. Later, however, exposure to the radiation had shrunk the animals enough for them to get out of their cages. 

In 2273, Montgomery Scott and Transporter Chief Janice Rand used gossamer mice to confirm that a malfunctioning transporter had been successfully repaired and could transport living matter. Eight people and 2,000 small Abaris natives were in dire need of rescue on Abaris.