Searching for a Cygwin package named telnet, I was surprised to be unsuccessful. This has a simple reason: telnet is contained in a utilities package called inetutils.
Follow these steps for installation:
- Run your Cygwin’s setup.exe and follow the all steps up to the package selection.
- Here, type inetutils into the search box. An appropriate match should be located below Net.
- Select inetutils for installation and install it the usual way.
- Problem solved thanks to this post.