MacTerm — 

Powerful replacement for macOS Terminal, supporting 24-bit color, standard graphics protocols and iTerm2 image sequences and color schemes.  Written by Kevin Grant.

Version 5.0 alpha (macOS 10.15+) in development, currently only available by building from GitHub.  More Info…

To run on 10.14 or earlier, please check out version 4.x.

See also the original telnet client for Mac Classic.