Olive said:
Again, technically, you can't apply two inherited templates like that, but it's your game!
Strictly speaking, you can.
First, let's check the templates themselves. The half-dragon template can only be applied to a living corporeal creature, and changes the creature's type to "Dragon". Conversely, the half-fiendish template can only be added to a living, corporeal creature with an Int of 4 or more and a non-good alignment, and changes the creature's type to "Outsider".
Strictly speaking, nothing in those rules them out for a manticore. Being a lawful evil Magical Beast, with an Int of 7, you can add either of those templates, in any order, and nothing rules any of them out.
What next? Well, looking at the "Multiple Templates" section of
Savage Species, we see that in terms of the "type pyramid", Magical Beast is outranked by Dragon, which is itself outranked by Outsider. Basically, that means that whatever order you add the templates in, the final type for the creature will be Outsider, since it's highest on the pyramid (in this case it is).
Lastly, in terms of having two inherited templates (in this case, having two "half-" templates), it's certainly possible, and is allowable by the rules. The "Templates and Biology" sidebar on page 142 of
Savage Species says, in it's last paragraph, "When adding multiple templates, halves become quarters. A creature with both half-celestial and half-dragon templates becomes a quarter-celestial/quarter-dragon creature." Just change "celestial" there to "fiendish" and you pretty much have your manticore!
