You are such a _____. You ate that whole pizza by yourself.填一个动物名称。

1. You are such a pig. You ate that whole pizza by yourself.

Pig can describe a person who eats too much.
e.g. You greedy pig!

2. My computer is missing! I think I smell a rat.

'Smell a rat' is an idiom. It means to recognize that something is not as it appears to be or that something dishonest is happening. 怀疑；发觉不对劲。

e.g. Jane's been working late with him every night this week - I smell a rat!
Jane 最近常与他工作地很晚，我怀疑有不正常的事情发生。

3. The girl is beautiful. She is a ___.
She could be a kitten, a lamb, a bird, a butterfly or a fox.
形容女孩的美， 可不止一种动物。

Sometimes attractive women with a particular facial configuration are described as "feline."
Audrey Hepburn was often called fawn-like, and consequently liked to be photographed with deer.

Hope this helps.

Cheers

Note: 本人已在海外生活了数十载。为了慎重起见，我的答案都与老美或老英交换过意见，你放心使用吧。如有任何问题，可补而述之。

pig猪（能吃）
rat/m