Frogs often rep spiritual transformation. The symbolism is their natural change from eggs in a sack into tadpoles & then into frogs that both swim in the water element & hop around on land, the earth element. Both earth & water elements have passive energy applications which supports the spiritual associations.
Toads have their own historic presence within the Craft for the toxins they secrete & there is a toad-bone ritual for obtaining a specific toad bone from a specific species (not just any toad) that is believed to augment psychic ability. Obviously the toad dies in the gruesome process.
Aren't they associated with Hekate and her possible origins as an Egyptian goddess?
I am the Earth, The Sun and the Stars
And I am the also the Moon
I am all animal and birds,
And I am the outcast as well, and the thief
I am the low person of dreadful deeds,
And the great person of excellent deeds
I am Female. I am Male and I am Neuter. - Devi
Since you mention it I went looking & found a long write up on Hekate at angel fire website regarding Her liminal facets & copied this: All wild animals are sacred to Hekate & She sometimes appears three headed as dog, horse & bear or dog, snake & lion. But the creatures of darkness & of the earth are most sacred to Her; ravens, owls, crows, snakes & dragons. The frog, significantly a creature that can cross between two elements, is also sacred to Hekate and the Egyptian goddess Heqit.
I hadn't associated frogs with metamorphosis before this. Apparently my high school biology class was either too long ago, or did not sufficiently impress me.