As far as I can tell, CHKDSK has been part of DOS and Windows since day one. However, in later versions of DOS, CHKDSK was shipped along side ScanDisk. As of Windows 98, running CHKDSK will result in the program printing a recommendation for ScanDisk instead.
In later versions of Windows (such as XP), ScanDisk had disappeared entirely, having been replaced by CHKDSK.
That said, why did Microsoft originally make ScanDisk, and why did they scrap it? Did ScanDisk do something that CHKDSK couldn't do? The Windows 98 version of CHKDSK says "SCANDISK can reliably detect and fix a much wider range of disk problems." Was this true? If so, what additional problems could ScanDisk fix?