you find your Shelgon in the Wetlands Area. when you first get the Blocks upgrade you can only throw down 30 pieces of Rocky Objects (Mossy Rocks, Tiny Rock), which means you're nowhere near getting a Shelgon to come out. You need a total of 63 Rocky Objects in the Area to call out a Shelgon.
if you leave that Area out in the Safari zone for 30 days, Rocky Objects receive that upgrade, and every 1 piece will now count for 2. So those 30 Rocky Objects will now count for 60... which is still not enough. You're gonna need to wait another 40 days to receive that second Upgrade... which every 1 rocky object will count for 3.