Lassie is the titular character in "Bianca's New Pet." At first, she is never shown, and because she is the namesake of a famous collie from an old movie and television franchise, Bianca assumes her to be a dog. Lassie is Bianca's reward for helping around her uncle's farm.
Because Bianca thinks the name proves Lassie is a dog, the plot is driven by planning on building a doghouse (in which process Bianca hurts her fingers hammering, and because of the slope on the roof, the hammer falls onto her toes).
When Bianca's doghouse is finished, Lassie is brought home, to her confusion, as instead of a dog, Lassie is a pig.