It is an avocado! And it is huge!
Apparently this is a Florida Avocado, which has a lower fat content than a typical hass avocado (by weight, not by volume). The little sign in the grocery store said to choose one that had a smooth green skin, which was essentially all of them. I'm not really sure how to know if it is ripe or not, so I just went ahead and tasted it right away.
I had been planning to have avocado on toast, because that is a thing I've heard of. And I really like avocado. And toast is not bad. Then I remembered that avocado and a poached egg on toast is a thing, so I made that instead.
Sidenote: That was the most perfect poached egg I've ever made.