Actually, I think that the bluish-gray Fast Biter might have been Screech, since they both are the same color. However, I don't think that the purple one is Thud, since Thud is green. My hypothesis is that the purple one in LBT11 was Screech's mate or love interest, whom he later "broke up" with. He then found Thud, who later became his best friend and companion in the TV series.