"In the highlands I have also heard that they use another form - red dog
(madra). Seems like we are supposed to use any other word than the 'real'
one - whatever that is?"
This is a cautionary practice (non-practitioners may call it "superstition")
especially among sailors, because saying the true name of an unwanted
thing -- "rat" on a boat, for example -- makes it come. That's why you don't
whistle on a boat: it brings the wind. I've been told contemporary true
stories by a fisherman on the Isle of Man about this. A rat can safely be
called "long tail" or "ringie" from the rings round its tail.