Who Matarishvan is
Matarishvan is the one who fetched fire. The Rigveda has no hymn to him and names him only twenty-seven times, nearly always in a hymn that belongs to Agni. But the few lines add up to one clear deed. Fire was hidden, far away. Matarishvan brought it, and gave it to people.
The verses agree on the outline. Agni was born in the highest heaven and was shown first to Matarishvan (1.143). He lay concealed, and Matarishvan rubbed him out of hiding, as fire is rubbed out of wood (1.141, 3.9). He carried him from far away as the envoy of Vivasvat (6.8). He brought him for Manu, the first man (1.128), and handed him to Bhrigu like something precious (1.60).
Like a treasure, Matarishvan brought the glorious priest as a gift to Bhrigu.
One verse sets him beside another famous carrier. Matarishvan brought the one down from heaven, it says, and the falcon tore the other from the mountain (1.93). The one is fire and the other is Soma, the sacred drink. The two things the old sacrifice could not do without were both fetched from somewhere else, and both by a bringer.
Who or what Matarishvan is, the poets leave open. In three places the name is plainly a name of Agni himself, as if the fire had brought itself (3.5, 3.26, 1.96). One poet explains the word: the fire is called Matarishvan when it has been formed in its mother, and then it becomes the rush of the wind (3.29). A famous verse says that the wise give many names to what is one, and lists three of them: Agni, Yama, Matarishvan (1.164). In the wedding hymn he is asked, with other gods, to join the hearts of the bride and groom (10.85).
Scholars have not agreed either. Macdonell thought he was a heavenly form of Agni, probably the lightning, imagined as a person who carries fire down. Oldenberg thought he was simply the Rigveda's Prometheus, a fire-bringer and nothing more. Those are their readings. The hymns themselves say that someone brought the fire from far away and that his name was Matarishvan.