中译英：使他们惊奇的是他们抓到的不是一只小猫，而是一只小老 虎。not, but

To their surprise,which they caught wasn't a cat,but a little tiger.我觉得是这样翻译的

So that they surprised that they caught is not a kitten, but a small tiger

what makes them shocked is not that they have catched a kitten, but a small tiger

What surprised them is that they caught is a little tiger but not a kitty.

What surprised them is what they caught is not a kitty, but a litte tiger.