Actually, "djur" is the plural form! It's an ett word, which means it will add an "n" if it ends in a vowel, and nothing if it ends in a consonant. Since 'r' is a consonant, the plural form is just "djur." You differentiate by whether it has an article in front of it. If it says "ett djur," it means "an animal." If it just says "djur," it means animals. "The animals" would be "djuren."