No, the sentence "It rained the animals took shelter under the tree" is not a complete sentence. A complete sentence expresses a complete thought and has a subject and a verb. The sentence "It rained the animals took shelter under the tree" has two independent clauses, but they are not joined together correctly. The first clause, "It rained," does not have a subject. The second clause, "the animals took shelter under the tree," does have a subject and a verb, but it is not joined to the first clause in a way that makes sense.
Here are some ways to correct the sentence:
* It rained, and the animals took shelter under the tree.
* The animals took shelter under the tree when it rained.
* When it rained, the animals took shelter under the tree.
The correct sentence will have a subject and a verb in each clause, and the clauses will be joined together in a way that makes sense.