This thing has been updated and it is a real pain when you find it in your logs. It is now "Pcore-HTTP/v0.24.5" and from 14/Oct/2016:20:24:32 to 14/Oct/2016:23:41:56 it requested 0ver 2,900 image files from 150 different IPs. I have added this UA to my go away list.
Pcore is one of a large group of robots that I only became aware of after adding files to a reputable directory: robots that don't do top-to-bottom crawls beginning with the front page, but only request files that are linked from elsewhere. Since they have never asked for robots.txt, I have never had occasion to poke a hole for them, and hence don't even know which particular header, or lack thereof, leads them to be blocked.
If it weren't for the request pattern--typically 6 requests for various files in the same directory within about 30 seconds--I might have suspected it's yet another open-source UA that anyone and everyone can use.