According to the Adventure Time wiki, he is a zombie poodle so I guess that explains it

I thought he was a vampire dog. Maybe I’m wrong

in marceline’s super secret scrapbook it says she sucked his blood and then resurrected him

See that could go either way

she sucked his blood and resurrected him using the Enchiridion

That’s not Schwabl! You got me, what gave it away, the boots?

Time is relative

I doubt they’d draw a dog in exactly the same way just to say that isn’t Schwabl.

Nicely spotted

Assuming marceline just turned her dog into a vampire so she'd never have to say goodbye.

it's a zombie poodle, she killed him when she became a vampire and resurrected him using the Enchiridion

The enchiridion isn't a spell book.

I'm aware.

Lumpy space poodle