Learn French: Ils croient que les animaux sont des monstres. - They believe that the animals are monsters.

Croire (to believe) is an irregular verb. In the present tense, its stem uses an i for the singular forms and the third-person plural (ils croient). It only switches to a y for the nous and vous forms (nous croyons, vous croyez).

Can I leave out the word que? In English we usually just say "They believe the animals..."

No, you must include it. In English, the word "that" is often optional. In French, you can never skip the conjunction que when connecting a verb like croire to the rest of the sentence.

Why does the sentence use des monstres instead of just monstres?

In English, we do not use an article when talking about indefinite plural nouns (we just say "monsters"). In French, nouns almost always need an article. Since we are talking about plural monsters in an indefinite sense, we must use the plural article des.

What is the singular form of animaux?

The singular is un animal. This sentence is a great reminder of a common spelling rule: most French nouns ending in -al change to -aux in the plural.