What id the adjective in the sentence This animal likes to swim?

There is none. An adjective is a 'describing' Word that is usually connected to a noun (in this case that would be the Word "animal") and is used to give that noun a specific quality. An example: had your sentence been: 'This beautiful animal likes to swim", 'beautiful' would be the adjective, because it describes a specific quality of the animal. The Word 'this' is not an adjective, although in a way it also describes what animal you mean. But we call a Word like this, that, these a demonstrative pronoun.