Why my AWS have too many public snapshots?

I am getting small amount of bill on AWS, even I m on free tier, I got billing on RDS. I reviewed my RDS but found no instance, as I deleted it earlier. I found a snapshot which I deleted. After couple of days I reviewed my billing and still found increment. I went through the RDS dashboard and in public snapshot tab, I found 223 snapshots which I did not created. I also found snapshot that were create on 2014, and at that time my account didn't exist. I do not know should I delete all 223 snapshots or are those some system related stuff.

Public snapshots is a way to share snapshot with other aws account publicly.

According to Docs

You aren't billed for the backup storage of public snapshots owned by other accounts. You're billed only for snapshots that you own but If you copy a public snapshot, you own the copy. You're billed for the backup storage of your snapshot copy. If you create a DB instance from a public snapshot, you're billed for that DB instance.

Note: You can delete only the public snapshots that you own. To delete a shared or public snapshot, make sure to log into the AWS account that owns the snapshot.

To pinpoint which service is causing those extra dollars check out this answer -> How to view all the services running on AWS?