Casper
The subject of the sentence is who or what the sentence is about. The sentence is about Casper. You can also identify the subject by asking who or what is doing the verb. In the sentence, the verb is caught. Who caught? Casper did. Casper is the subject. The other words in the sentence are as follows: "a" and "purple" are adjectives, "butterfly" is the direct object. A direct object receives the action of the verb.
In some sentences, the subject is implied. These are commands. For example, "Sit down." This sentence implies the subject as you. As in "You, sit down."