Unix

On page 44 of his book Unix: A History and a Memoir, in the context of a photo showing Thompson and Ritchie next to a PDP-11 around 1972, Brian Kernighan briefly mentions an experimental portable terminal built from a modified Teletype Model 33 teletypewriter. The keyboard and printer were in a suitcase-like container weighing 55 pounds (25 kg), with a built-in acoustic coupler for dial-up connections. Kernighan used the portable terminal from home a couple of times.

PLATO

Although not a commercial product, at CDC Jock Hill developed several prototypes of a portable PLATO terminal.

The devices, which for years were carried around the world for demos, featured an 8"x9" 512x512 orange glow plasma display, a keyboard, an acoustic coupler, and a phone headset for dialing into PLATO. These terminals became available in the early 1980s but I have no information on the exact timeframe.

Source: Chapter 22 "The Business Opportunity" (Part III) of the book The Friendly Orange Glow, The Untold Story of the Rise of Cyberculture by Brian Dear.