Nyancat rendered in your terminal.
First build the C application:
make && cd src
You can run the C application standalone.
./nyancat
To use the telnet server, you need to add a configuration that runs:
nyancat -t
... to either an inetd or xinetd server. I am using openbsd-inetd, which you must give both arguments (nyancat and -t) to, which I found odd.
The original source of the Nyancat animation is prguitarman.
The code provided here is provided under the terms of the NCSA license.