They are trying to figure out how you think and what motivates you. As said in a previous answer, complete frankness is a double-edged sword and I won't repeat what IIy333o said. But what's more important is your attitude toward these types of questions in general and I've been guilty of thinking of them the wrong way as well.
Don't see this question as an interrogatory as much as an opportunity to share some personal insight about yourself. Remember, interviews are almost all about getting to know you as a person and see if your attitude and motivations are a good fit for their team. They're not really a goal review.
In my experience, the one quality that helps most in interviews (both as an interviewer and the candidate) is passion. It is passion (and any emotion or personality for that matter) that is missing from your resume. People don't interview to find out your skills (if they're doing it right). They have that information from your resume/CV. They bring you in because they not just want to get to know you but to give you the opportunity to "wow" them. And nothing wows people like a genuine passion, whatever that may be.
So, how to we get that passion across without telling them, "My dream job is something you can't offer"?
I would slightly move away from the question itself. While they asked for your dream job, tell them about your dreams and your passions in a more abstract way. For instance, you could start by telling them about your two great passions in life, animal welfare and programming. Tell them why animals are such a passion for you. Then talk about your programming passion. Wrap it up by merging the two but not so much as a dream job, but a dream. You could say something like "I don't even know if it's possible to even combine those two dreams in a job, but I don't want to give up one over the other, so for me it would be a dream to use my programming skills to help animals someway such as volunteering to help animal welfare organizations in my spare time so that one passion aids the other"
If pressed, you can finish with something I've said previously like "I do have dreams, but it's not just a job. For me, a great job that helps me also pursue my other passions IS part of my dream. My dream job is a great job. This looks like it could be a great job."
The key (and I can't stress enough) isn't what you tell them as an answer to the question, but what you tell them about yourself and what drives you.