Favored Enemy: Outsider(evil) is slightly confusing to me personally because when you look under fiends and demons the succubus is under both. Which demon is under outsider(evil) is this an exception or are fiends technically demons at least under the benefit of favored enemy, or vice versa? If they count as demons do half-fiends as well?
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Demons are a subset of fiends.
All evil outsiders are fiends. Chaotic evil fiends are demons.
Having evil outsiders as your favored enemy means you get the bonus against all fiends and all demons (because fiends == evil outsiders, and demons are a type of fiend).
The half-fiend template changes the target creature's type to outsider, but does not appear to give it the evil subtype, so you most likely would not receive your favored enemy bonus against a half-fiend.
All monsters have a Creature Type, which can be found near the top of the stat block. Some types, such as Humanoid and Outsider, always have a subtype as well, indicated in (parenthesis).
"Fiend" is a flavor term used to describe a multitude of alignment-evil Outsiders, not all of which have the (Evil) subtype; a specific example being the Rakshasa, which is of the Outsider (Native) type.
Favored Enemy selections indicate which Creature Type the Ranger receives bonuses against. Only those monsters with that specific type (subtype) count.
The Half-Fiend template changes the creature's type to Outsider, and its alignment to "always evil."
However, from page 291 of the Monster Manual 3.5, under Templates:
Then, from page 140, under Half-Fiend:
The example creature is a Human, and his type has been changed to Outsider (Augmented Humanoid)(Native). This would suggest that no, Favored Enemy: Outsider (Evil) would not be used vs. a half-fiend. Similarly, the "Fiendish" template does not even grant the Outsider type.