I have done a bit of research and this is the only partial answer I can find for you. We all know that the Douglas Clan Crest is of a green salamander surrounded by fire sitting on top of a chapeau. While I have no direct proof to why the Douglas' chose a salamander, there is quite a bit of mythological and historical reasons than we can make inferences upon.

In western tradition, each element has its own mythological creature associated with it and the salamander was first connected with the element of fire. This belief/ myth was spread throughout Europe that "fire salamanders" lived in logs. When these logs were brought in to be burnt for warmth or cooking these mythical salamanders would scatter from the fire. (Salamanders lived in rotting logs since it was their preferred environment.) Since these salamanders would always run through the fire to escape, the common belief that the skin of a salamander can resist fire persisted even to modern time. The chapaeu in the crest represents the fact that owners of a barony were given a chapaeu or "cap of maintenance" as part of their armorial achievement. This was described as "gules doubled ermine" for most barons in possession of the caput of the barony.

With this in mind, the Douglas Clan Crest can represent a number of things for the clans people. The Douglas were well known as fierce warriors that survived against the odds. They usually came out of most conflicts on top. Seeing as how the salamander was coming through the fire on top of a chapaeu, this can represent the Douglas warriors overcoming the barons they came into conflict with during their many, many battles. It can also be suggested that the Douglas' chose the salamander to reflect their own resiliency in tough situations as well as a chapaeu showing their rise to martial and political power through marriage into royalty.

This is the best answer I can think of with the given information. I hope this helps loudbeak!

*EDIT* This is from David Arthur Douglas on Facebook. I believe his answer holds more historical weight and accuracy. But I would still like to think my answer was close =)

The Heraldic signification of the salamander, A Heraldic Beast", was that of a brave and generous courage that the fire of affliction cannot destroy or consume.

The salamander has been immemorially credited with certain fabulous powers. The skin was long thought to be poisonous, though it is in reality perfectly harmless; but the moist... surface is so extremely cold to the touch that, from this peculiar quality in the creature, the idea must have arisen, not only that it could withstand any heat to which it was exposed, but it would actually subdue and put out fire.

A golden salamander is so represented on the garter-plate of James, Earl of Douglas, K.G., the first Scottish noble elected into the Order of the Garter, and who died 1483 A.D. Tinctured vert; and in flames proper it is the crest of Douglas, Earl of Angus