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A Coleonyx brevis
An amphibian was a species of cold-blooded vertebrates. They spent part of their time on land but were still required to immerse themselves in fresh water the other half of their time.
A female sentient amphibian was a member of the Elysian Ruling Council. (TAS: "The Time Trap")
Deanna Troi de-evolved as an amphibian
The Comic once told Data, when asked to define what was "funny", that along with "Tip O'Neill in a dress" he personally found "a briefcase that looks like a fish" to be hysterical. When Data accessed his database on the subject, he reiterated the description as a "20th century male, politician, overweight, wearing female clothing, carrying a valise that looks like a fish," and took in to mean that "[t]he juxtaposition of gender and an amphibian briefcase is funny." (TNG: "The Outrageous Okona")
When Counselor Deanna Troi was affected by Barclay's Protomorphosis Syndrome in 2370, she de-evolved into an amphibian, complete with gills. (TNG: "Genesis")
When Tom Paris underwent a phase of celerity-induced accelerated somatic mutation rate after reaching Warp 10 in 2373, he rapidly evolved into an amphibian lifeform resembling a salamander. (VOY: "Threshold"; LD: "We'll Always Have Tom Paris") Anthony experienced a similar transformation. (LD: "Much Ado About Boimler")