I was thinking it was a deer also and maybe our dog spooked him off before he finished his meal. We have a lot of blacktail deer here. They eat everything, and our landscaping reflects their appetite.
Yeah, I'd guess it took the first test bite from the front right (hence the gills left intact) and then a more confident chomp from the left. Both are from an upper angle, and the teeth seem far too large for anything smaller that might nosh on rando fungi (raccoons, squirrels, rabbits, etc.), so your local blacktail are my bet. 🤓
I'm not a mycologist, but it looks a bit like a pavement mushroom I found in Norway. But that looks bigger. Probably something like agaricus arvensis, but I'm not a mycologist and only have a slight interest in the field. The real answer is it's probably an agaricus of some type or another, but don't eat it.
Thanks for your input and narrowing it down to agaricus. Agaricus augustus seems more prevalent in our area and matches the description on local online sites. This species is apparently edible but I'm afraid this particular one is well past its prime picking time.