You ask if we can just say "I wish I was a butterfly" instead of the subjunctive form "I wish I were a butterfly." You actually can often get away with it, but not if you have listeners who are knowledgeable and more discerning about English grammar. You see, a subjunctive sentence is one that denotes unreal acts or states that are contingent on possible outcomes of the speaker’s wish, desire, or doubt. Such sentences stick to the past-tense subjunctive form “were” all throughout, regardless of the person and number of its subject. In contrast, the sentence "I wish I was a butterfly" is in the indicative mood, which means that a human being turning into a butterfly is a real-world possibility--which of course it isn't.
As if you've read my mind, I have actually scheduled a series making a full-scale review of the subjunctive by the second half of this month (January 2018). Watch for it!
