The difference between the two based on the "Cambridge dictionary" article, is as follow (quote):
"We use "it", "this" and "that" to introduce further information about a topic already mentioned. However, the words have different uses.
We use "it" to continue to refer to the topic we are already writing or speaking about:
The heart is the central organ in our bodies. It is used to pump
oxygen around the body through the bloodstream. (It refers back to The
We don’t use "it" when we first give information about a topic, for example immediately after a chapter or section title in a text:
(b) Green application form
*This* must be signed by all applicants and returned by 30 November 2009.
Not: *It* must be signed …
We can use "this" to refer back to whole clauses and sentences and to previous parts of a text. "This" highlights the information referred to much more strongly than it. Writers often use "this" when a point or idea is to become an important part of the discussion that follows:
More and more people are discovering that Tai Chi is one of the most
valuable forms of exercise. This has led to a big demand for classes.
(This refers back to a whole sentence.)
Heavy rains and stormy conditions throughout the summer have led to
severe shortages in strawberries and other soft fruits. This has led
to price rises in many supermarkets and shops.
We use "that" in a similar way to this. However, when we use "that", we distance ourselves more from the topic or from aspects of the topic:
For many traditional football supporters, it is a problem that so many
young girls and women attend football matches these days. That is a
sexist attitude of course.
"That" is also used to refer to ideas associated with another person:
The chairman apologised for the poor performance of the company and
promised a better future for investors. That was a promise many people
felt he could not possibly keep." (all of the above from the Cambridge dictionary. I've just added some graphics such bold letters etc. in order to make it easy for reading and understanding).
The above Cambridge article deals with the using of "it", "this" and "that", while writing or talking generally. But the original question is also about a case of pointing on an animal or a fruit for example and talking about it. So in that case should it be "It is a nice animal" or "this is a nice animal"?
It seems that it depends on context. If we are around with the ability to point on the object, meaning that we see it then we can use "this" or "that" depends on the distance. But if we don't see it at all, we use "it".