I'm currently developing a fantasy setting, where at least two of my sapient species will be hexapods(six limbed animals). One is a species of quadrupedal "giants", using four legs(among other stuff) to support their larger size, and the other is a race of tree dwellers that use the extra limbs to easily move through the trees while still having multiple limbs free. They're probably going to be similar to mammals, but that could change.
I could just go with the simple reason of "feesh has six fins", but I don't want practically all of the animals in my setting to have six limbs. I need some reason for these animals to develop an extra set of limbs somewhere down the line. Does anyone have any relatively plausible ideas for how this might happen?
Also, I believe that my planet will be pretty similar to earth for the most part, so it wouldn't come about due to something like abnormal gravity. Anyways, thanks for responding, if you do, and have a nice day!