Retards defined as, I don’t consider Forest Gump to be a retard. He was intelligent, able to communicate, skilled at combat, etc.
Retard is more like, that drooling crazy guy mumbling at you, eating at his own flesh, that you have to point away from oncoming traffic.
My cat seems to be able to communicate with me quite easily, I can sense how she is feeling, she seems to have interests such as looking out the window for friends, she can hunt, cuddle, play, she holds grudges, and we both have that awkward silence when we look at each other too long we have to look away because it feels awkward. My cat is also fond of mozart and dislikes bad music. My cat appears more sentient than many humans. I have a larger brain than her, so it appears sentience is related more to brain structure and key functions, rather than size alone (for instance, an AI having a large vocabulary would probably be less sentient than an animal with a small vocabulary.)