The first pianos were made in around 1700. Researching a little online, I've been able to find lots of info on the history of the piano. But when was the "baby grand" piano first created and/or marketed for sale?
Maybe the German word for "Baby-Grand" helps you with your search: "Stutzflügel". If German Wikipedia is correct, then
Der erste Stutzflügel nach heutigem Größenverständnis als Neukonstruktion, d. h. ohne einen längeren Flügel zu stutzen, wurde von der Ernst Kaps Klavierfabrik AG im Jahr 1865 gebaut.
In English: "The first Baby-Grand (which was not made by cutting off a big grand) was built in 1865 by Ernst Kaps Klavierfabrik AG".
In 1865, Kaps built the first double overstrung grand piano. ... Previous attempts at producing a successful small grand piano failed until this time and subsequently, the Kaps Double overstrung 5 ft grand piano, quickly became a revelation. Hailed as the first successful small grand, later known and promoted by another maker as the "baby grand piano", launched Kaps pianos into commercial success.
Then Sohmer must be the “other who promoted and manufactured”... I found this “In 1884, Sohmer created the first baby grand piano, the first smallest grand pianos ever manufactured at only five feet long. The company also patented the agraffe bars and actions in 1882, in 1887 an agraffe for a quadruple strung "reverbation scale" and a pianissimo pedal in uprights, and bridge agraffes in 1890.”