The Dog Tag is a quest item.
It's shaped like a red dog collar with a gold bone-shaped tag and belongs to the Robot Dog, having been left behind after the dog went missing. Before Mickey goes into Tomorrow City, Horace asks him to locate the missing tag as a lead in his case. The Tag is located in Space Voyage behind a glass case to the right of Mickey's starting position. Mickey has to thin out the surface of the nearby elevator (the one that leads up to the platform with the pad that, when activated by a TV Sketch, makes the crane operational), ride it up, get out, and jump onto the platform that faces the opening of the case to get the Tag.
After Mickey gets back, he gives the Tag to Horace, who presumably uses it to solve the case and rescue the dog.
Trivia
- Throughout the game and guidebook, it's referred to in it's plural form "tags," even though it appears to be just one. Plus, a dog only wears one tag.
- The collar is possibly based off of Pluto's old red collar. In recent pictures, he has a green collar.
- For that matter, the main item isn't even a tag; it's a collar!
- A brown version of the col- er, tag, is the appearance of the Mystery Solved Pin, the Pin you get for completing this quest.
- If Mickey doesn't get the tag and deliver it to Horace, he can grab it when he revisits during the Battle in Space Voyage. Mickey presumably takes the tag home if he does so, since at that point he can't return to Mean Street to give the tag to Horace.