rxvt

In the early 1990s xterm was the only serious option, and it carried the whole X Toolkit, a Tektronix 4014 emulation nobody used and a memory footprint you could feel on a 386 with four megabytes. John Bovey at the University of Kent wrote xvt as a stripped-down alternative, and Rob Nation, who went on to build FVWM shortly afterwards, took the code and extended it into rxvt, "our xvt". It spoke plain Xlib, skipped the Tektronix mode and used a fraction of the memory. In late-1990s Linux distributions it was often what actually opened when you clicked the terminal icon.

The descendants say something about the era. aterm gave AfterStep pseudo-transparency, Eterm did the same for Enlightenment with more sparkle, mrxvt added tabs. The branch that survived is rxvt-unicode, urxvt, which Marc Lehmann started in 2003: full Unicode, Xft fonts, embedded Perl for extensions and a daemon mode where a single process serves every window. The original stopped at version 2.7.10 that same year, but the name lives on in every ~/.Xresources that begins with URxvt.