What motivated you to become a veterinary assistant? Are you interested in the field but want to get your feet wet first? Think about how this job fits into your long term goals. Here's an example of how you can respond:

"I'm interested in becoming a vet tech at a zoo. Learning from you and your staff will help me to acquire the skills and experience I need to move forward in my career."

You can keep it simple. No need to discuss all of your future goals. Just share your main career focus. Do some research on the company to see how their values, mission or even types of clients will help you to attain your goals. This will also help you to assess if they are the right company for you.