Flag of the Sultanate of Delhi according to the Catalan Atlas (1375). The flag is grey with a black band in the Catalan Atlas (attached image), not green with a black band as previously uploaded. The actual grey and black color appears clearly in the primary source (and the Catalan Atlas has plenty of green otherwise, so the grey cannot be a result of color fading), and this interpretation is confirmed by academic sources:
REFERENCE: "....that helps to identify yet another curious flag found in northern India – a brown or originally sliver flag with a vertical black line – as the flag of the Delhi Sultanate (602-962/1206-1555)." in  (2010). "On the Timurid flag". Beiträge zur islamischen Kunst und Archäologie 2: 148.
WARNING: This is primary source image, the actual flag is otherwise unknown. The main source pointing to this design is the Catalan Atlas, which as a historical primary source. This image should not be added to articles without clearly attributing its primary source origin, and secondary sources are additionally desired if available (such as the one above).  Note that FOTW (where most of those flags on Commons are adapted from) is based on user contributions like Wikipedia, and hence not authoritative.OVERWRITING OF FICTIONAL FLAG:

The file of the actual grey flag from the Catalan Atlas was overwritten by the uploader over the previous green flag, a hoax that actually cannot be found in the Catalan Atlas. The original file claiming to be the "Flag of the Delhi Sultanate" being a hoax, it was quintessentially false and misled users, and therefore had to be updated with a proper and accurate image. This is following the spirit of the COM:OW rule about "Files with current data" which for example allows to modify the file of a map which has false national boundaries: "Files with current data: (...) However, files may be identified to reusers as ones where information may be updated – these files can and should be overwritten to reflect new information as necessary. (...) A file named "File:Germany location map.svg" is expected to indicate the current boundaries, and is expected to be updated if, for example, the boundaries changed or errors were observed." So a map with false boundaries can, and should be, overwriten: similarly a national flag with a false design can and should be overwritten.
Own work
LG02, पाटलिपुत्र (talk) for the grey and black version.