Lewis & Short

Asturco, ōnis, m. [Astur], an Asturian horse, an ambler, distinguished for the beautiful motion of its limbs (cf. the epigram, Mart. 14, 199, and Sil. 3, 336), Auct. ad Her. 4, 50; Sen. Ep. 81: Equini generis, hi sunt quos thieldones vocamus, minore formā appellatos Asturcones, Plin. 8, 42, 67, § 166; Veg. 2, 28, 37.
Transf. to other horses possessing similar qualities: Asturco Macedonicus, Petr. 86.