To play, all you have to do is pick a pet from your collection that can beat the pet posted before it.
The order goes like this:
(Lowest) OMGSC < EC < VC < C < UC < VUC < EUC < R < VR < ER < OMGSR (Highest)
*Dates do not matter*
However, here's a twist: OMGSRs can only be beaten by OMGSCs and ERs can be beaten by either OMGSRs or OMGSCs.
You don't need to follow the order perfectly. For example, any pet on a higher rarity tier than a common can beat one. Posting a pet with the same rarity is also acceptable.
Ex:
Person 1:

Person 2:

or
Person X:

Person Y:

Have fun!