And the last time you saw one of these was...
most likely, never.
This is a show and tell item:
Two of them are about the size of a golf ball, plus one slightly smaller.
The texture is different on each one.
I think the texture of this one is the most interesting.
These lovelies came from the inside of one of my mom's Arabians.
They are called "Enteroliths", or more simply: intentional stones.
The common belief is that they are made up of minerals: magnesium, ammonium and phosphate. Most often found in Arabians or mares (these were from an Arabian). There is more information and good preventative advise here: Enteroliths