Description in Films
My Little Pony Equestria Girls
Spike the Dog is first mentioned by Pinkie Pie when she asks Twilight Sparkle if she had a twin sister with a dog who looks like hers.
Equestria Girls: Rainbow RocksAfter the closing credits, Spike the Dog appears on-screen alongside human Twilight eating some dog food from a bowl. Unlike Spike the Dragon, Spike the Dog only barks.
Equestria Girls: Friendship Games
Spike the Dog appears along with his owner Twilight in the third film. As he was depicted in the post-credits scene of Rainbow Rocks, this Spike is depicted as being non-sentient and only able to communicate like an actual dog. Both Rarity and Fluttershy recognize him when he pops out of Twilight's backpack. Spike shows much concern for Twilight throughout the film, as he dislikes Canterlot High School and expresses worry about Twilight's trip there.
When Twilight's pendant absorbs Fluttershy's magic, causing dimensional rifts to Equestria to open, Spike follows a jackalope through one of the rifts, and he is accidentally exposed to some of Fluttershy's magic. This gives him the ability to speak and communicate with humans, though he assures Twilight he hasn't been affected in any adverse ways.
During the climax of the film, when Twilight transforms into the evil Midnight Sparkle, Spike calls out to her and gives a pleading look, momentarily bringing her back to her senses so Sunset Shimmer can return her to normal.
Spike is present when Twilight's Equestrian counterpart appears from the portal, happily wagging his tail at the sight of a second Twilight Sparkle.
Equestria Girls: Legend of Everfree
In the fourth film Legend of Everfree, Spike accompanies Twilight, her friends, and their class of Canterlot High students to Camp Everfree, expressing a desire to chase squirrels. Similar to Sunset Shimmer, he serves as a source of comfort and support for Twilight Sparkle as she deals with recurring nightmares and worries about Midnight Sparkle.
At one point in the film, Spike has a dream about chasing squirrels, and he briefly realizes this dream during Legend You Were Meant To Be.