i think you are right : i used to feed only super rare for my baby account and the knitten kitten there is level 8 , so they must have changed rarity for it. go figure why since there is already a rare animal dark-earth (dreambat) . probably because it is the only one (the are 2 super rare dark earth : gargolem and pigmy giraffe)
edit : i looked in the creatures library, lonepudding started the thread for knitten kitten (1-5-2016) and knitten kitten was super rare