What and how much is known about it?
It is a rare but widely distributed motif on both intaglio seals (Mohenjo-daro) and moulded tablets (Harappa). Facts about each object include its archaeological context (if that context is known), what other depictions occur on the same artefact (associated motifs and "script"), what the artifact is made of, and what its shape is. In spite of much speculation in the literature, we have no actual idea of the meaning or significance of this image, because there is no way to verify any interpretation, especially in the absence of a widely accepted decipherment of the "script". Furthermore, an unbroken (without significant temporal gaps) line of use of the motif from the Indus period into the historical period has not been demonstrated.
I. Mahadevan’s (1977) Table VIII, Distribution of Field Symbols by Site, on pp. 777 to 795 of his Concordance, shows that field symbols 46-49 seated horned personage and animals/serpent or personage by himself occurs ONLY at Mohenjo-daro. He is shown by himself on three seals, With the rhino, buffalo, etc. on just one seal, and with a “kneeling adorant” and a rising snake on two seals
I don’t know if recent excavations at Harappa have found this motif on any of their seals [yes, they have ]--- if not, this is extremely significant for the role of Mohenjo-daro, don’t you think?
In addition to this famous "proto-Shiva Pasupati" seal there are a few other seals and tablets with a similar ithyphallic figure crowned by a bull- or buffalo-horned headdress and seated in a yogic pose, often flanked by various combinations of fishes, gharial, snake and other creatures or associated with a worshipper or a scene probably from mythology. All were found at Mohenjo-daro. The iconography and associations of the figure strongly suggest that it represents a deity with many of the attributes of the later god Shiva.
I recommend Asko Parpola's Deciphering the Indus Script (Cambridge University Press, 1994) for discussion of this figure and of Harappan religion in general.