Malemunyon at Unfiction has found the origin of the mysterious Robot shirt.
It looks like the text underneath the robots reads "GIANT ROBOT". Giant Robot is a bi-monthly magazine of Asian and Asian American popular culture. While they currently don't have this shirt for sale in their store, a similar design can be found at Polar Panda.
No one still seems to be 100% sure what the robots are, but it looks like it may be Transformers after all, based on this ad found by mixedtapex16.
To celebrate the find, I have put all the Slusho Robots shirts on SALE.