[dn3500]

Square pixels were certainly not an innovation of the Macintosh. The earliest raster scan workstation I'm aware of is the Alto, released in 1973. It and the ones that came after it like the Star and Dorado all had square pixels. So did the early 1980s engineering workstations like Apollo and Sun, which also came out before Macintosh.

[rob74]

I think the Amiga is the most well-known example of what OP means. Older home computers which could be connected to TVs generally had resolutions up to 320x200 (or x240 for PAL) and square pixels. The Amiga could double that on both axes to 640x400/480, but because of the interlaced display of typical TVs/TV-based monitors, that would flicker so bad that it made productive working impossible. So the default resolution used by AmigaOS was 640x200/240, and the fonts were optimized for that.

[Gormo]

Even in the PC world, the most common resolution in for CGA, EGA, and VGA remained 320x200 for many years. With square pixels, this would be 16:10, but the usual case was that this resolution was displayed fullscreen on a 4:3 display, so individual pixels would have an aspect ratio of 5:6.

Most DOS-era games took this into account, so e.g. if the artist wanted to draw a circle 20 pixels tall, they'd make it 24 pixels wide. Textmode followed this pattern as well, so when rendered on a modern square-pixel display without aspect correction, will look vertically squashed compared to their original appearance.

[musicale]

But the Lisa, whose developers included PARC alumni who had moved to Apple, had rectangular pixels.