Scared frog is drop from Adult Goanna and from Young Goanna .
Its a part of Kilmaresh Quest.
Also you need one to Achivement :
Sculptor Apprentice
Granted, you didn't carve those lifelike animal figurines yourself. But helping a medusa to find proper objects and even watching her using her petrifying gaze is almost as rewarding.
2 points (Secret)
Give Alyxo a bat, a scorpion, a scared frog, a calamary, and a small tortoise.
And you need to finish first mission in Kilmaresh quest, the same for enter to boss Urmah~