i'm partly making this thread to try to track some of the patterns with pets that are available in the pound on a rotational basis based on pets that are moved into the "adoptions" group. i'm wanting to see what patterns there are and what odds there are on an individual basis to get a specific rarity, based on what pets are in the group at a given time. 
!!! UPDATE !!!
FrostedGalaxies has made a Google Sheet to compile data as well! link
feel free to reply to this thread with what pets/math you've calculated at a particular point in time (if it hasn't been already documented in the thread for that time period). of course, none of this math is definite - it's speculative, trying to figure out if there are any patterns to pets being added to the adoption group and if there is any consistency in the rate at which a potential rarity can be seen.
pets in the "adoptions" group as of 12/28 9:30 PM EST:
extremely rare: 10 (0.74%)
very rare: 39 (2.9%)
rare: 84 (6.24%)
uncommon and below: 1,118 (83%)
unknown rarity, including dec. 18th pets: 96 (7.13%)
total count: 1,347
there are two store pets in the adoptions group, giving a 0.15% chance of receiving one.

!!! UPDATE !!!
FrostedGalaxies has made a Google Sheet to compile data as well! link
feel free to reply to this thread with what pets/math you've calculated at a particular point in time (if it hasn't been already documented in the thread for that time period). of course, none of this math is definite - it's speculative, trying to figure out if there are any patterns to pets being added to the adoption group and if there is any consistency in the rate at which a potential rarity can be seen.
pets in the "adoptions" group as of 12/28 9:30 PM EST:
extremely rare: 10 (0.74%)
very rare: 39 (2.9%)
rare: 84 (6.24%)
uncommon and below: 1,118 (83%)
unknown rarity, including dec. 18th pets: 96 (7.13%)
total count: 1,347
there are two store pets in the adoptions group, giving a 0.15% chance of receiving one.