This photo of "terrible monster" was taken during the walking to the forest in Stare Drawsko, beautiful lakeside village near Czaplinek, Poland.
Biological name of this frog is European tree frog in English, Rzekotka drzewna in Polish language or Hyla arborea in latin.
This species is extremely rare and under absolute Polish law protection.
NO ANY FROG WAS HARMED DURING MAKING THIS PICTURE! :)
PS: WS added with improved quality!