Think of it this way:
You and I are humans. Here, individually each of us is a human.
We are human. Here we're talking about the human race, and it's no "s".
"Họ thích động vật" is the same way as "human." It refers to the whole animal kingdom. If you go to the zoo and you say "I like THESE/THOSE animals," then you would need to use những or các to demonstrate plurality. Does that make sense?