Learn Spanish (Latin America): Había muchos animales invertebrados en el bosque. - There were many invertebrate animals in the forest.

When haber is used to mean 'there is/are' or 'there was/were', it is impersonal, meaning it has no grammatical subject. Because of this, standard grammar rules dictate that it always stays in the third-person singular form, even when referring to plural items like muchos animales. While you will sometimes hear native speakers say habían muchos animales, keeping it singular (había) is the required form in standard Spanish.

Why do we use the imperfect había here instead of the preterite hubo?

We use the imperfect había to set a scene or describe a continuing state or background information in the past. This perfectly fits describing the animals existing in the forest over an unspecified period. You would use the preterite hubo to declare that a specific, bounded event took place, such as a storm or a festival (for example, Hubo una tormenta en el bosque).

Why does muchos come before the noun, but invertebrados comes after?

Adjectives of quantity like muchos (many) or pocos (few) are limiting adjectives, and they almost always go before the noun they modify. Meanwhile, descriptive adjectives that classify or distinguish the noun from others, like invertebrados, follow the standard Spanish rule and go after the noun.