A collie was a breed of dog originating from Scotland and England on Earth. This breed was used for herding sheep.
When the crew of the USS Enterprise was trying to convince the cosmic cloud not to consume Earth, images from the ship's library computer were shown to the non-corporeal cloud creature, and one of those images showed the silhouettes of Human children running in the country with a collie. (TAS: "One of Our Planets Is Missing")
The collie was none other than Lassie, the star of Lassie's Rescue Rangers, as the images were reused footage from that series.