rlogin
BSD's remote login from the early 1980s, which trusted the machine you came from instead of asking you for a password.
rlogin shipped with 4.2BSD in 1983 alongside its siblings rsh and rcp, collectively the r-commands. Unlike Telnet it passed along your username and terminal type automatically, and if the calling host was listed in /etc/hosts.equiv or your .rhosts, you were simply let in. On a campus network where everyone knew everyone, that was convenient. It ran on TCP port 513 and was documented after the fact in RFC 1282.
The catch was that the trust rested on an IP address. Kevin Mitnick's attack on Tsutomu Shimomura on Christmas 1994 spoofed exactly such a trusted address to get in via rsh. SSH arrived the following year, and the r-commands became something you disable during install.