yes it is possible , they often burrow under the grass or come out after or during the rain... so yes quite possible.
On a website it says to flush the mouth out with running water using a hose or kitchen sink spray hose.. wipe the inside out the mouth several times using clean clothes or different corners of a towel, making sure not to use the same corner twice. Give them a 1/4 teaspoon of pepto to settle the stomach. A little peanut butter in the mouth will get rid of the poison taste from the frog. I have my own little prep kit in the kitchen for the " just incase". But happy your off to the vet.
Symptoms of Bufo Toad Poisoning in a Dog | eHow.com