Looking at my monstergram to see how many of my rares are ranked. So they break down as follows:
Rank 0=4 (Gravoid,Garuda M3, Fornax, Aurinia)
I can understand Gravoid being at 0. That is one UGLY monster.
I can also understand Vapwhirl, Eggknock, and Hedgy being at Rank 5 because I tried to get them there. What I find bizarre is that in the process of just playing the game, I also got 6 other rares to rank 5.