SatyaVeda: All about the Veda

Agnih Devata:

Devata means luminary, the one who is bright. The root of the word is related to light. Diva is lamp, Diva is day, Divi is heaven. Even in English the word divine meaning Godly has its root in the word light. (Some others have translated Devata as angel or demi-god etc., but that does not apply to the Vedic Context, it was an attempt to reach the Vedas through a different religion which has the concept of angels).

In modern Indian languages and in modern sanskrit, Agni means fire, literally.

In Vedas and Upanishads, Agni is not only the fire that burns wood, but also the fire of the digestive juices, the fire of lightning, the fire in the sun and so forth. The nearest english word that strikes me at this point is energy.

Agnih Devata: The luminary is Agni (energy).

From this we may understand that the Prathama Suktham will be presenting the well said words by Vaisvamitro madhucchanda, a rshi about Agni Devata, in the Veda that is about Rk (Arka).