Press-Enterprise: Sixteen years have passed since wildlife biologist Kevin Brennan watched a flying squirrel leap from the deck of his Idyllwild home, extend the furry flaps along its sides to create a sail, and glide effortlessly to the snowy ground. At one time, such a sight was common in the San Jacinto Mountains. But not anymore. Brennan is believed to be one of the last people to glimpse the novel creatures in the mountain range between Palm Springs and Hemet. “It didn’t register with me at …