我要一猫 is a wrong sentence which needs quantifiers. In this case, you should use '只' to describe cat. Then the sentence is 我要一只猫 that also has difference from 我想要一只猫。 The mood of first sentence is stronger like you are giving an order or wanna buy a cat in the shop. The second sentence means you wish to have a cat and it's more polite when you are saying to an assistance. The key words are '要'，'想要'.

1. if someone has a cat already, and you want to get it from him, you could say: i want a cat from you. 我想向你要一只猫。 2. if there is no such background, you just simple tell someone you want a cat, I would say: 我想养一只猫。 养(yang3)=raise. hope it would help.

Hello Alex , I think you should use " 我想要一只猫" . 1- because the classifier for cat "只" is used here and missing in the other sentence. 2- also because "要 " means rather to demand, request and ask for something ,while " 想要 "expresses the desire to have and fancy something and that reflects the meaning better in this context.

我要一猫 is wrong, 我想要一只猫 is right. Eventhough Chinese have no grammar, but it has its own logic in accepting what is right and what is wrong. In Chinese, a single word may have many meanings. So, it has to be two words to make it unnique. 一只 is a unique number.