Beginner Gardening: Is this mushroom poisonous? my Golden Retriever tried to eat, 1 by Mojoquilter

Mojoquilter wrote:
Hey all, I posted this in the plant identification forum but just in case, I wanted to post it here because my friends in the beginning gardening forum are so knowledgeable and quick to reply. I let Mac out this morning and noticed him with something white in his mouth so I went right out to see. He had one of the three that grew overnight. I got it away from him and tossed all 3 over the fence but now I'm worried. I can't find any pictures that look like this to see if it's poisonous.