It would cycle through a small number of text values with some kind of backspace/overwrite to keep things localized to where the cursor ought to be.

One version was a variable length ellipses: . .. ... that would grow and reset in place.

Another was an expanding "dot": . o O that would cycle in place as one character.

And the early "spinner" was: - \ | / that would cycle in place as one character. Hmm, not sure this will render properly on HN but it is hyphen, backslash, pipe, forward slash.