As I discuss in this answer, how the Vedas originated is that from time immemorial, sages known as Dhristas have heard sacred verses directly from the gods during a state of Tapasya (deep meditation). And then in the Dwapara Yuga, a sage named Krishna Dwaipayana Vedavyasa (or Vyasa for short) compiled these verses into a set of four books we call the Vedas. (Technically Vyasa only compiled the Rig, Yajur, and Sama Veda; the Atharvana Veda was compiled by the sage Atharva and Angiras).
In my answer here, I compiled information from the Anukramanis (indices) of the Rig Veda, specifying which sage heard each verse from the gods. As you can see, one who heard most of the hymns in Rig Veda Book 7 is the famous sage Vashishta. (His last name is Maitravaruni because he was reborn as the son of the gods Mitra and Varuna.) But two of the hymns of Book 7, hymn 101 and hymn 102, are attributed to "Vashishta Maitravaruni or Kumara Agneya."
So my question is, who is this Kumara who may have heard Book 7 Hymns 101 and 102 from the gods? His last name is given as Agneya, or descendant of Agni the fire god, so I think it's possible that it's the god Kartikeya (AKA Skanda or Muruga). This may seem strange at first glance, since Kartikeya is the son of Shiva, not Agni. But if you remember the story of Kartikeya, as described in this chapter of the Bala Kanda of the Ramayana, Kartikeya was born from Shiva's fiery seed, which was carried by Agni and then deposited into the Ganga river. As a result, Agni is often considered as an adopted father of Kartikeya.
So are there any scriptures that describe Kartikeya as the seer of two hymns of the Rig Veda? The hymns themselves don't provide much of a clue; they're just dedicated to Parjanya god of rain:
- Speak forth three words, the words which light precedeth, which milk this udder that produceth nectar. Quickly made manifest, the Bull hath bellowed, engendering the germ of plants, the Infant.
- Giver of growth to plants, the God who ruleth over the waters and all moving creatures, Vouchsafe us triple shelter for our refuge, and threefold light to succour and befriend us.
- Now he is sterile, now begetteth offspring, even as he willeth doth he change his figure. The Father's genial flow bedews the Mother; therewith the Sire, therewith the son is nourished.
- In him all living creatures have their being, and the three heavens with triply-flowing waters. Three reservoirs that sprinkle down their treasure shed their sweet streams around him with a murmur.
- May this my song to Sovran Lord Parjanya come near unto his heart and give him pleasure. May we obtain the showers that bring enjoyment, and God-protected plants with goodly fruitage.
- He is the Bull of all, and their impregner: he holds the life of all things fixed and moving. May this rite save me till my hundredth autumn. Preserve us evermore, ye Gods, with blessings.
- SING forth and laud Parjanya, son of Heaven, who sends the gift of rain May he provide our pasturage.
- Parjanya is the God who forms in kine, in mares, in plants of earth, And womankind, the germ of life.
- Offer and pour into his mouth oblation rich in savoury juice: May he for ever give us food.
The only connection I could find is the fact that the Vishnu Purana says Parjanya is the presiding deity of the month of Kartika, when Kartikeya was born. But are there any scriptures describing Kartikeya worshipping Parjanya?