Shiny mod question.

Me and my friends are trying out the shiny mod on extinction for a few days now. But so far the most interesting shiny dino was a snow owl. The rest was only simple creatures like anky, doedi, thorny dragon etc. The timer for shiny dino to appear is set on 15 minutes and max 10 active. So there is a stready supply of shinies every 15 minutes.

But the thing is, we have still never seen a rex, managarmr or any other high attack dino appear as a shiny. Is this blocked by any setting that i may have overlooked?

Hope somebody can help me out here.