It is an instrument from the harpsichord family, but smaller and with a different format. The strings are in an inclined position relative to the keyboard.
Spinet by John Player (17th century) - Victoria and Albert Museum - London - England
It has the same mechanism that harpsichord, the sound is produced by martinetes that pluck the strings.
The spinets, like the harpsichords, were usually imported to Rio de Janeiro. In late 18th century, the Customs New Tariff for the Rio de Janeiro fixated the prices of "smaller harpsicords or spinets" in 11$200 reis.
The only known surviving example of a Portuguese spinet is currently in the Imperial Museum of Petropolis. Built by Mathias Bosthem in 1785, the instrument belonged to José da Cunha Porto and was donated to the National School of Fine Arts, in Rio de Janeiro, in 1902, which later transferred it to Imperial Museum, where it is until today. The instrument image is illustrating the top of this page.
Watch the video below with Rosana Lanzellote playing an excerpt from João de Souza Carvalho's Sonata.