about the definite article
"My favorite animals are monkey and dolphin."
Is this a correct sentence? Thank you.

Question

Answer

Normally, we speak about animals in the plural:
'My favourite animals are monkeys and dolphins.'

Answer

Presumably, you like all monkeys and all dolphins (or at least the majority), so you should use the plural form. If you are being specific, stay singular; if talking about a group, be plural:

My favo(u)rite actors are Jackie Chan and Jet Li.
My favo(u)rite colo(u)rs are red and blue.
My favo(u)rite countries are China and Japan.

My favo(u)rite animals are cats and dogs.
My favo(u)rite cars are expensive (ones) and fast (ones).