BA (Hons) Computer Animation Arts, UCA
for me, there's something in the idea of the animal's perception of the vet that is interesting in story terms - it's a simple idea - zoo animals are terrified of 'The Executioner' - it's a nice set-up and it makes sense. I think your story could be as 'simple' as setting up this terrifying myth amongst the animals, and showing how one animal is taken away by this shadowy terrifying figure.. the trick is to make the audience share the subjective horror of the animals... you then set up a third act twist when it is revealed that the animal that was taken away is in fact being looked after and is absolutely fine - perhaps a bunch of animals go to rescue the other one, all guns blazing etc - only to realise their mistake... of course, if you were going to go very dark, you could let this story play out, with the animals storming the executioner's lair in the dark - we hear the sounds of screams - and all the animals are victorious - and then the scene is revealed as they and the audience understand they've just killed and eaten... the vet.
I've always been a fan of "bad" endings, where the heroes lose... and equilibrium is achieved through the bad guys taking over and ruling. Like the end of Starwars Episode 3, the empire wins... sure we know they are beaten later on but the ending of that film is a dark one.Maybe through a little bias but i feel like the animals should lose... perhaps the vet seems nice to us but it soon becomes apparent that he's messed up, deranged and cruel. The animals fight but lose... and the "vet" does unspeakable things to them...