Too big for a wildcat or a marten, i think.
It could just be badger... They can produce solid stools if they've been scavenging a carcass rather than eating worms.
It could simply be from another dog though... Is the area where you found it public?
A real conundrum this one!
No public goes where this was founf, well not that i know of. I would say badger but theres not a set close by. Puzzled