Queen's was addressed as such, and considering that Jack's has his "human" upper body bar the trunk, I'm quite convinced that centaur form is supposed to be his hybrid. "Hybrid" can be assumed as a stable form where both base and beast form are combined reaching the overall body; only partial transformations like Pekom's, Orochi's or Onigumo's, in which an isolated body part is changed while the rest remains the same, are harder to clasify. Jack's only abnormality are his four mammoth legs, but the rest of it sounds classic hybrid to me.