Either Bob or Horatio was adopted.
Yep. There is so little resemblance between them that it's hard to imagine that they actually came from the same parents. Maybe Horatio was the adopted one, which would possibly explain why he keeps a posh accent while living in Tennessee.El Diablo in the climax...
For a while I thought his hybrid form must be half-bat because of his wings. But then I realized, there is no bat cookie shown in the movie, but there is a dragon cookie at the very end. This is fitting for El Diablo's skill as a fire breather. Horatio's accent is fake.
Considering how melodramatic Horatio can be, it could be possible that he originally had an American accent as a kid and that his British accent was something he adopted during his career and kept afterwards.