One off

You describe something as a "one off" if it is original, it may be a prototype of a new car, when only one has been made. A person is a "bit of a one off" if they are different or an original thinker, apart from the crowd.

The cat is a "one off" in that if it genetically engineered and neutered then there is only "one of a kind". There may be a pun implied in what is "off" , ie what has been removed in the neutering process, I'm not sure without seeing the full thing.

Here is the word used in context: http://www.btinternet.com/~oneoffjewellery/