So if an aasimar Divine Hunter worshiped an evil God and had a fiendish pet and tookt he celestial servant feat.
Would your pet be both fiendish and celestial?
How would these stack? Would you have dr good and evil?
nothing?
I suspect most GMs would disallow being both fiendish and celestial, but going strictly RAW it is not forbidden, so....
You would end up with DR X/good and DR X/evil (which mostly works out to DR X/(good and evil), though I vaguely think there's some border case where it differs). You would have (non-stacking) energy resistance to acid, cold, fire, and electricity. You could smite evil 1/day and smite good 1/day. You'd have darkvision 60' and SR of CR+5 (since neither of those stack).
Enough stuff doesn't stack that I'm pretty dubious it'd be worth the feat, but if you decide otherwise and your GM doesn't mind, enjoy your wacky pet! :-)