It depends which region of the country you are in. In some places it is common for Callery ornamental pear trees to pollinate pear trees in orchards. Animals will then grab the fruit and spread the seeds, which will grow up to be a second generation of hybrid trees that produces fruits. These fruits often taste inedible but the seedlings can become invasive and spread in the wild like weeds. Pear trees that are growing on their own roots, not grafted, can grow to become quite large, especially given several decades of time.

If it is simply sour and astringent but otherwise seems to have good it could be a perry pear (variety used for making pear cider) but I think that is unlikely since perry hasn't been very popular in America for over a hundred years.

It's also possible it could be a regular pear that grew from seed, possibly spreading in the wild. They might not taste quite as good as regular pears, since growing from seeds can shuffle around the genetics a little bit.

do you know how to ripen pears... ???

dad used to just lay them on a picnic table in full shade ... after they fell ... and after a few weeks or a month or so.. they would finish ripening so they werent rock hard ... and the sugars developed ... to be clear.. this is in MI ...

i always presumed they must have been good little rocks to put in the root cellar.. to be enjoyed into the winter months ... but alas.. who has a root cellar anymore ...

also.. as long as the skin is intact.. forget about the external uglies.. just use a paring knife to remove it.. and try the flesh ... once it softens a bit ...

lastly.. the usual invasive pears dont make fruit that large... so i figure its not that .. but it still might be a seed grown plant that just doesnt taste good ... there are reasons to buy named cultivars.. to insure the crop is useful ...

ken