And here’s where it takes a weird spin – Alternate reality/dimension. The dominant life form is the centaur (horse/human hybrid). The centaur use the humans of the world as their slaves/serfs. Feudal society – with king, queen, prince/princess, etc.
So – humans stand out and are looked down upon by the larger four-footed intelligent creatures. There are plain horses as well, but those are treated like distant cousins rather than something inferior.
The bad guy/girl is an evil sorcerer/scientist.
The portal between the worlds moves around. So the boy (street rat/gang banger?) knows where he came in, but when they get there to check it out- it’s gone. They figure out the sorcerer/scientist creator? person is behind the portals – so they travel to that person’s fortress to seek a way home.
In between, there’s a story about a young centaur maiden who captures the eye of the king – whose had a few wives – think Henry VIII.