Shasha is the Saturn member of the class, and Saturn's qualifying signs, Capricorn, Aquarius and Libra, give it the same three-sign basis as Mars, Jupiter and Venus. The name is glossed as the hare in most treatments, and as with the swan of Hamsa the animal names are traditional labels whose reasons are not always explained by the sources that use them.
The class is set out in Varahamihira's Brihat Jataka, one of the foundational Sanskrit handbooks of the sixth century, and in the Brihat Parashara Hora Shastra, the large compilation that later tradition treats as its principal authority and whose date of composition is itself a matter of scholarly disagreement. Where the two differ in detail, later commentators generally follow Parashara.
That disagreement about dating is worth flagging rather than smoothing over. Brihat Parashara Hora Shastra survives in manuscripts and printed editions that vary between themselves, and it is not securely datable as a single composition; treating it as an unproblematic ancient authority is a habit of popular writing rather than a finding of textual scholarship.
The two texts differ in emphasis as well as in date. Brihat Jataka is compact and treats the yogas as one topic among many in a work of a few hundred verses; the Parashara compilation is far longer, more systematic and more concerned with dasha timing, and it is from the second that the modern practice of testing every combination against its planetary period principally derives. Reading the class through one text or the other gives noticeably different weight to timing.
Sources Brihat JatakaVarahamihira with Brihat Parashara Hora ShastraSage Parashara (attributed), Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree