Probably a squirrel if in the daytime. At night, rats, raccoons, and opossums are my best guesses.
It is sad to see a good mango, or any fruit, go to waste.
It is probably best to pick up the fallen fruit and dispose of it asap so as to not attract more competition and pests.
I try to keep our squirrels and birds (and who knows what else) well fed, and away from the fruit trees. Seems to help a lot moving the feeders around depending on what is fruiting.