Cubone is messed up, for real.
The Detective Pikachu movie is great at making the Pokemon universe feel like a place that could really exist. There's a whole lot of backstory from the Pokemon movies and TV show sprinkled throughout (not to mention just a ridiculous number of Pokemon hanging around) that gives the setting weight, and the movie's self-awareness helps build the sense that people really do live in this place. All that knowledge of other Pokemon properties also offers a host of cool callbacks and Easter eggs for Pokemon fans, and a great deal of world-building for those who aren't as familiar with the franchise.
But Detective Pikachu's self-awareness also means it's willing to make jokes about some of the weirder bits of Pokemon lore, like the backstory surrounding one cute Pokemon who may be the world's saddest: Cubone.
Early on in the movie, Tim heads out in an attempt to capture a lonely Cubone with his pal Jack. As Jack notes, capturing a Pokemon is as much about the Pokemon choosing to team with the human as it is the reverse, so Tim tries to talk to the Cubone and gain its trust, and makes a comment about the skull the Cubone wears on its head: the skull of its dead relative, as Tim notes. If you're unfamiliar with Cubone's backstory, that sounds a bit weird, but the actual lore surrounding the Pokémon is even weirder.
Tim's right: Cubone wears the skull of a dead relative as a hat. In fact, that's the skull of its dead mother. The circumstances of how every since Cubone's mother died is never discussed, but it's seemingly a pervasive tragedy. Wearing the skull doesn't seem like a great way to deal with that trauma, to say nothing of how extremely morbid and fairly gross it is.
To make the whole thing a little sadder, here's the official Pokemon Company Pokedex entry for Cubone, which is the one that appears in Pokemon Ruby and Sapphire:
"Cubone pines for the mother it will never see again. Seeing a likeness of its mother in the full moon, it cries. The stains on the skull the Pokemon wears are made by the tears it sheds."
Yikes. In the movie, Jack jokes that Cubone would be a great Pokemon for Tim because they're both lonely, but that is a lot of baggage. It gets even more intense.
In the movie, Tim and Jack find the Cubone because of its mournful cries (which echo in the skull and come out as a sad melody, according to Pokemon Yellow), which Jack said his mother previously heard. Pokedex entries in Sun elaborate on that even further: The crying sounds are how a Cubone predator called Mandibuzz finds its prey. So not only is Cubone constantly sad and crying, but those cries will often lead to it getting eaten by another Pokemon.
The Moon entry at least adds a bright spot, suggesting that Cubone can heal from its trauma (if it survives!) and get over its mother's death, stating that when it comes to terms with the tragedy, it will evolve into the larger, tougher Marowak. But rather than just go about its life finally happy, Marowak trades its sadness for a new drive: vengeance. Sun and Moon's Pokedex entries say Marowak goes after revenge on Mandibuzz--which suggests Mandibuzz killed its mom in the first place.
One last tidbit: Sun and Moon added special versions of first-generation Pokemon to fit the games' tropical Alola region, and the Alolan Marowak gives a slight bit of additional context. Though Cubone's mom is dead, she's not actually gone, according to the Pokedex:
"The bones it possesses were once its mother's. Its mother's regrets have become like a vengeful spirit protecting this Pokemon."
Just to sum up, Cubone wears its mom's skull on its head, its cries of mourning get it attacked by a predator, it finally deals with its loss and evolves into a revenge machine, and it gets help from its dead mom's spirit in its never-ending battle with Mandibuzz.
Guess it makes sense that Detective Pikachu didn't want to get much deeper into the ridiculous Game of Thrones drama surrounding Cubone. Hopefully that Cubone who didn't feel like getting caught by Tim is okay.
Disclosure: ViacomCBS is GameSpot's parent company