A violoncello piccolo is a violin-sized instrument tuned like a cello. It is held horizontally, slung from a from a strap over the shoulder lika guitar.
Some of Bach’s cantatas specifically written for this instrument.
It is possible that Bach wrote the cello suites for the instruments (no instrument was specified on the manuscripts).
Source: Siblin, Eric (2009) The Cello Suites. Crows Nest: Allen and Unwin, p. 251.