What's the Name of That Book??? discussion

I’m looking for a graphic novel that I swore was called Necropolis, but I can'€™t find it by that name. It’€™s about a kid who somehow finds himself in this alternate skeleton/under world.
SPOILER:
I remember something about a plane being the portal into the skeleton world.
I also remember one scene where he’€™s still in his normal world and a skeleton horse appears and gets stuck in the wall. I think once he was in the skeleton world, it was a rush for time so he didn't turn into a skeleton too...

Maybe Ghostopolis by Doug TenNapel? It's not Necropolis but it's close!

It came up searching "graphic novel" "skeleton horse" "a portal".

I think that’s it! Thank you!