C- = Common, Cat 1
C = Common, Cat 2
C+ = Common, Cat 3

R-- = Rare, Cat 2
R- = Rare, Cat 3
R = Rare, Cat 4
R+ = Rare, Cat 5 (Space Being Dragon)
R++ = Rare, Cat 8 (Dreamcatcher Dragon)
R+++ = Rare, Cat 6 (Underworld)

VR- Very Rare, Cat 3 ( Ukulele and Twin)
VR, Very Rare, Cat 4
VR+ Very Rare, Cat 7 (Illusion, Prophet)
VR++ Very Rare, Cat 6 (Alien Pet, Bark)
VR+++ Very Rare, Cat 5

E- Epic, Cat 4 (there's only one dragon in the game like this, not sure what it is)
E Epic, Cat 5
E+ Epic, Cat 6
E++ Epic, Cat 7 (Rideable)

L- Legendary, Cat 6
L Legendary, Cat 5
L+ Legendary, Cat 9
L++ Legendary, Cat 10

H, Heroic, Cat 11

What do you think?