Yes, Leopard has telnet built-in, both client and server although the server is stopped for obvious security reasons.
You can run the telnet client by opening up the Terminal app (found in Applications -> Utilities) and then typing "telnet" or the telnet connection command you need.
An alternative way to connect to a telnet server is to open up Terminal app, then from the menu choose "Shell -> New Remote Connection" and enter your telnet server connection details in there. This is for 10.9 but I am pretty sure it was the same in 10.5.
Telnet is inherently insecure and I really wouldn't advise using it across public networks.