Learn Spanish (Spain): El animal malo es lento.

They are describing el animal. Because animal is a masculine singular noun (which we can tell from the word el), any adjectives describing it must also take the masculine singular form, ending in -o.

How do I know animal is masculine when it doesn't end in -o?

Not all Spanish nouns follow the -o and -a rule. For words ending in a consonant like -l, you often just have to learn the gender when you learn the word. Luckily, the masculine word for "the" (el) right before it gives it away!

Why is the order animal malo instead of malo animal?

In Spanish, descriptive words (adjectives) like malo usually come after the thing they are describing. So "the bad animal" is phrased as "the animal bad" (el animal malo).

Why do we use es here instead of está?

Both mean "is", but we use es (from the verb ser) to talk about permanent traits or what something is normally like. Using es tells us this animal is naturally slow, not just temporarily moving slowly right now.