There are a lot of ways to make a creature undead.

• Animate Dead: Only works on humanoids.
• Create Undead: Only works on humanoids.
• Finger of Death: Works on everything, but will only turn humanoids into undead.
• The Death Tyrant's Negative Energy Cone: Works on everything, but will only turn humanoids into undead.
• The Shadow Dragon's Shadow Breath: Works on everything, but will only turn humanoids into undead.
• The Shadow's Strength Drain: Works on everything, but will only turn humanoids into undead.
• The Vampire's Bite: Works on everything, but will only turn humanoids into undead.
• The Wight's Life Drain: Works on everything, but will only turn humanoids into undead.
• The Wraith's Create Specter: Only works on humanoids.

None of these methods will create a non-humanoid undead. However, as @keithcurtis points out, the Monster Manual includes a large number of undead that aren't made from humanoid creatures.

• Beholder Zombie
• Bone Naga
• Death Tyrant
• Dracolich
• Minotaur Skeleton
• Ogre Zombie
• Warhorse Skeleton

So clearly it is possible to have non-humanoid undead. So why are there no published methods to make one? Well, I have no proof, but I believe it's to ensure that players can only make undead using the standard statblocks. If, for example, Animate Dead worked on the corpse of any creature, there would either need to be zombie and skeleton statblocks for every creature, or some sort of standardized process for zombifying creatures. The first is pretty clearly unfeasible, and the second tends to be open to exploitation.

That's everything players have access to. However, for DMs, there is a large section on Creating Monsters in the DMG (page 273), including creating monsters by modifying existing monsters. For a Vampire Pixie, I'd recommend starting by applying the changes listed in the Player Characters as Vampires sidebar, Monster Manual page 295, and go from there. Other than their special abilities, which they probably wouldn't be able to use once they became Ghouls, Dryads are fairly similar to humanoids, so the Ghoul statblock shouldn't need much alteration to represent Dryad Ghouls.

1. Yes -- banishment says it is "a more powerful version of dismissal", and it doesn't override the 20% chance, so that is in fact how it works.

2. Given that your question is tagged "pathfinder", by the Rules As Written there is no way that the PCs could wind up in Maztica or Dark Sun or Greyhawk (as these are all D&D settings).

A very literal-minded DM might argue that, since no destination within the Material Plane is specified, the PCs arrive at an entirely randomly chosen location, which is overwhelmingly likely to mean they show up in airless deep space, far from any sun. Let's hope they have a silent greater teleport spell ready!

A less literal-minded DM might treat the dismissal effect as though the PCs had cast plane shift -- they'd appear "5 to 500 miles (5d%)" from the place they left the Material Plane.

The Pathfinder Wiki has some words about the Pathfinder settings that officially exist on the Prime Material Plane. If you're determined that the player characters arrive on the surface of a human-livable planet, but you'd prefer that it be a randomly selected planet, your choices include Golarion, Earth, Androffa, Kasath, and, er, Carcosa.

Of course, this is your cosmology; if you'd like to rule that multiple D&D settings exist in your version of Pathfinder's Material Plane, that is your prerogative.