European Grass Frog (Rana temporaria), that I found on a trip to the Mussala hut. This one and a few more where a bit darker than the normal and I don't know why. Sexual dimorphism? I doubt it. Maybe it's just about to shed it's skin, but I'm not sure.
And here is a normal one I found under a rug near the hut itself. I decided to take a few shots of it, too, just for comparison. :P