Corrected sentence (short and simple):
A peacock looks pretty but sings like a rusty door.
Clearer version (with comma and repeated subject):
The peacock looks pretty, but it sings like a rusty door.
Why these are correct (easy steps):
- Always start with a capital letter and end with a period.
- Use an article: say "A peacock" or "The peacock" so the sentence sounds complete.
- When you join two full ideas with "but" and repeat the subject, put a comma before "but": "The peacock looks pretty, but it sings..."
- "Sings like a rusty door" is a funny comparison. If you want it to sound more normal, you can use "sounds like a rusty door." Both are okay if you mean the peacock's noise is unpleasant.
Pick the first for a short sentence, and the second if you want to show both ideas clearly and follow the comma rule.