The Theme Is "W"

Well, Kind Of
More specifically, the theme is "Were-beast", which does begin with "W", so we are still in accordance with Working Backward From The Letter Z.  Which just kind of happened, if you want to know.
     Now, I know you're all desperately keen to know where the derivation - what's that?  You'd prefer to be going out to the pub and then a party? - SIT BACK DOWN!  You will be educated, if I have to come over and rivet your slingbacks to the floor.
     Now you've made me lose the thread.  Were was I?*
     Yes, the word "Were".  This is from Old English and means "Man", presumably the generic term for you Hom. Sap. in your entirety, because otherwise you'd need the word "Wowere" or even "Shewere" for the female of the species, although the word "Dere" might be apt, if likely to draw a bit of fire and ire from feminists, who would be really up in arms if Conrad used "Fere" instead, or indeed "Mere".  Which might also cause confusion as it means "Lake".
     Okay, I think I've thoroughly confused everyone, including me, so let the motley commence!

Were-wolf
Everyone is familiar with this trope.  From Lon Chaney to <thinks> Oliver Reed and <thinks even harder> Benicio Del Toro, even if Lon's version looked as if he was suffering from excessive hair growth.

Lon's hair-replacement therapy had - side-effects

     Conrad, ever one to ask awkward questions, wondered why this legendary creature evolved?  It doesn't really exist before the nineteenth century, so none of your "Ooooh the answer is lost in the mists of antiquity".  And, Conrad asks again, why a wolf?
     "Because they're so terrifyingly deadly ferocious monster carnivores!" you bleat in a knee-jerk response.
     Sorry, no.  I have looked into this and the Brown Bear is a far more dangerous European critter.  So, for that matter, is the Wild Boar.  These are genuinely dangerous creatures, whereas the wolf is merely this a few generations displaced:

Made up of Awww!

What alternatives to the Werewolf are there?  Let us research this matter.

Were-wrasse
Frankly this isn't too terrifying, especially when not in H2O.  A Wrasse is a type of fish, as this picture explains:

A fish with entirely too much LSD in it's design specs

     You might be expecting to have another picture here, one of a Were-wrasse. 
     Sorry.  Have a screenshot instead:

Tah-dah!

     Conrad is proud to have come up with a concept that has broken Teh Interwebz.

Were-Weasel
NO!  Conrad, for many years a sincere friend of the weasel, simply refuses to countenance this.

Here, however, is a Weasel SHARK.  With a man.  Close enough.

Were-wallaby
Now you're just being silly.

O rlly?

     Ahem!  - apparently not silly enough.

Were-wendigo
Aha!  Two supernatural entities for the price of one!

     I realise this is not the terrifying supernatural entity you were expecting, but bear with me.  This is Peter Pan.  Peter Pan, solo.  Never mind The Lost Boys, where - get this! - where did Wendy go**?

Were-warthog
I have to say this is an unfortunate were-beast, less terrifying or horrifying than disgusting.  Whilst it might have the potential to rip yer gizzard out, it's really more likely to empty your stomach rather than disembowel it.

     The one above might be an exception, due to it's hairstyle.
     Dreadlocks.

Were-wombat
Hah!  Gotcha!  You were thinking of the terrifying transformation of another Antipodean animal, this one - 

Another made up of Awwww!

     No, I meant - obviously! - this WOMBAT -

Preparing to deal with the Bolshevik hordes

Weapon Of Magnesium Battalion Anti Tank, of course. It won't work unless you've got a man in charge of it.

Were-wren
See the "Were-wallaby".

Sir Christopher Wren, anyone?
He appears to be a Were-carpet

* Do you see what I - O you do.
** Sorry.