1. I have a dog, Lucky. He is cute.
2. I have a dog, Lucky. She is cute.
3. I have a dog, Lucky. It is cute.
[Can we use all the expressions? Which one is commonly used?]
4. I have a dog who is very smart.
5. I have a dog which is very smart.
6. I have a dog that is very smart.
[Which relative pronoun do we have to use?]
When referring to a person or animal you must use who, whom, he, she, etc. We do not refer to them as an it or that.
I disagree with Lee. Only people are referred to as who. Animals are referred to as that or which.
1, 2, and 3 are all grammatically correct sentences and can be used interchangeably. The choice between "he," "she," and "it" depends on the gender of the dog. If the dog is male, you would use "he," if female, you would use "she," and if you're referring to the dog in a general sense, you would use "it."
As for 4, 5, and 6, all three relative pronouns ("who," "which," and "that") can be used to introduce a relative clause to provide additional information about the dog. It is generally accepted to use "who" when referring to a person and "which" when referring to an animal or an inanimate object. However, "that" can be used in both cases. So, in the given context, you can use any of the relative pronouns ("who," "which," or "that") to refer to your dog's intelligence.