[Nomenclature]

The reason the quien can be omitted in that is because the author is using an appositive which functions the same in both English and Spanish. An appositive is simply a second name or title for something that goes after it (in both English and Spanish). There is no need to use words such as "who" or "which" to introduce an appositive.

For example, I can say, "The artist Pablo Picasso was very prolific". In Spanish this is <<El artista Pablo Picasso era muy prolífico>>. Pablo Picasso is an appositive for "the artist". In English, if an appositive is essential in order to be able to understand the sentence we do not put it in commas (my sentence does not use commas because the reader needs to know I am referring to Pablo Picasso). If an appositive is not essential (like in your sentence) and just background information, then we use commas.

Of course, you can also use an adjective clause (which is a dependent clause that describes a noun, in this case it will describe Pablo Picasso) here instead of an appositive which is this, "Pablo Picasso, who was an artist, was very prolific" This sentence uses the subordinating conjunction (a fancy word that refers to words that start dependent clauses) "who". When we use subordinating conjunctions like "who or "whose" we can have a whole new clause with its own subject and verb. In this case, who is the subject of the dependent clause. "who" WAS the artist. But if you say "Pablo Picasso, whose house was on fire, was an artist.",
then "house" is the subject of the dependent clause. Adjective clauses follow the same rules as appositives regarding commas.

You might be thinking, "What is the purpose of dependent clause if I can just use an appositive?" Good question. The reason is that you can't always use an appositive. For example, "The dog who likes to run is chasing the ball." "Who likes to run" is a dependent clause modifying "The dog". We cannot use an appositive to convey this.

Here is an example of a dependent clause modifying a verb (a.k.a. an adverb clause) "If I go home before the party, I will be late." "If I go home before the party," is a dependent clause modifying the verb "will be".

There is one last example I want to show you that native English speakers never get right. In English, whom is the objective form of who (me is the objective form of I). Native English speakers will say or write something like this,"The principal, who I like, is nice." But this sentence should actually be, "The principal, whom I like, is nice." This is because even though the who or whom goes first (it has to introduce the dependent clause) we are really saying,"I like whom." in this instance "who" needs to be "whom" because otherwise you are saying, "I like he" instead of what you should be saying which is ,"I like him."

If you didn't understand the last paragraph, don't worry. That's really minor. Also know a sentence has to have at least one independent clause. The important thing is that you understand the paragraphs above that. Animal Farm is an awesome book. I loved it and am sure you will enjoy it.