Dear Howard,
If you mean the original sources that would have sexual contact between the queen and the horse,
well the original sources predate the genre of śāstra.
See the Āpastamba Śrautasūtra 20.18.4, Baudhāyana Śrautasūtra 25.29,Kātyāyana Śrautasūtra 20.6.16
Vārāha Śrautasūtra 3.4.4.15
Older than these, however, is Śatapatha Brāhmaṇa 13.5.2.2
and the oldest explicit case that I know of is Taittirīya Saṁhitā 7.4.19 which opens with
út sakthyòr gṛdám dhehi
If, however, you mean the earliest sources that recommend the queen *merely* lying by the head horse while omitting explicit details,
I think that might be the epics.
Best,
Caley