Well even if it's invisible, I can't seem to be able to trigger/activate the event on the 1st tombstone (the one behind the gym panel right?). I tried to click it but nothing happens. Unless I have to click on every tombstone "blindingly".
For Zangoose I tried every weather (well, thunderstorm, rain, windy, and clear) during night time and he doesn't appear in the lower peridot alley, since that's where he's supposed to appear the 2nd time, unless I'm very wrong.
EDIT: Invisible on the plant NEXT to the broken tombstone behind the gym panel. Wow okay.
EDIT: And I was very wrong. He was in the Lapis alley, so my guess that he appears randomly in the alleys during night. Thanks for the tip.