Starting with Windows Vista, Microsoft no longer includes the telnet command installed by default. It is still available but must be installed or enabled by the user. The normal way to enabled additional features is through the “Turn Windows features on or off” window found in Control Panel. This accomplishes the task but can be annoying when you are working in Command Prompt and need to use the telnet command right away.
Instead of going to Control Panel, there is a better way. At an administrative level command prompt, simply run:
dism /online /Enable-Feature /FeatureName:TelnetClient
Telnet will be available immediately after the dism command has completed running.
As pointed out in the comments, the feature name is case sensitive.