According to the Oxford dictionary,
IT is a PRONOUN (third person singular) – that is used to refer to:
1.A thing previously mentioned or easily identified.
2.Referring to an animal or child of unspecified sex
She was holding the baby, cradling it and smiling into its face.
3.Referring to a fact or situation previously mentioned, known, or happening.
Stop it, you’re hurting me
4.To identify a person.
It’s a boy.
5.Used in the normal subject position in statements about time, distance, or weather.
It’s half past five.
It was two miles to the island.
6.Used in the normal subject or object position when a more specific subject or object is given later in the sentence.
It is impossible to assess the problem.
She found it interesting to learn about their strategy.
7.To emphasize a following part of a sentence.
It is the child who is the victim.
8.The situation or circumstances; things in general.
No one can stay here – it’s too dangerous now.
9.Exactly what is needed or desired.
You either got it or you haven’t.
10.It (informal third personal singular) – sexual intercourse or sex appeal.
The only thing I knew nothing about was ‘it’.
11. It (informal attributive third person singular) – denoting a person or thing that is exceptionally fashionable, popular, or successful at a particular time.
The company is renting out the It bags of the moment for as little as £10 a week.
12.Usually ‘it’ third person singular (in children’s games) the player who has to catch the others.
- That’s it
That is the main point or difficulty.‘‘Is she going?’ ‘That’s just it—she can’t make up her mind.’’
That is enough or the end.‘okay, that’s it, you’ve cried long enough’
This is it
The expected event is about to happen.‘this is it—the big sale’
This is enough or the end.‘this is it, I’m going’
So, the short version is that we usually use ‘it’ when we talk about animals or inanimate objects.
Eg. This is a stone. It is heavy.
That is a flower. It is blue.
Although when we are talking about animals we use it, if we want to emphasize the special or personal relationship with a specific animal or if we want to show its sex, we can use he or she.
Eg. I saw a dog. It was big. (here there is no personal relationship with the animal)
I saw a dog. He/she was big. (here we are referring to the sex of the animal)
So, it is grammatically correct to use ‘she’ or ‘he’ to refer to animals. By doing this we can show our affection and we personalise the animal.
Eg. My cat is very friendly. She always plays with everybody.