I have counted the rarity of all 8223 pets on CS through 2017!!!
As of February 2018:
OMG So Common 20 (0.2%)
Very Common/Common 3,613 (44%)
Uncommon 2,807 (34%)
Rare 1,187 (14%)
Very Rare 570 (7%)
OMG So Rare 26 (0.3%)
By Year / OMG So Common / Very Common & Common / Uncommon / Rare / Very Rare / OMG So Rare
2008 0 0 2 47 155 8
2009 0 2 36 419 203 4
2010 0 17 508 230 43 4
2011 2 292 493 61 25 0
2012 3 520 219 52 28 0
2013 7 524 228 59 23 3
2014 5 607 339 58 22 1
2015 0 623 332 78 28 2
2016 3 549 303 103 22 2
2017 0 479 347 80 21 2
More CS count statistics later