The first and main paragraph here = Arrian, Parthica fr.36.
[1] i.e. in Syria. Mount Casius (Kasios), nowadays Jebel Aqra, overlooks the Mediterranean coast at the modern-day Syrian-Turkish border (Barrington Atlas map 68 grid A2). The Suda lexicographer is evidently confused about the location of this prominent mountain; the mouth of the Orontes River (omicron 622) is indeed 20km to the north, but the Euphrates (epsilon 3810) actually flows some 150km away in eastern Syria. (The addendum to the entry, below, concerns its better-known homonym near Pelousion (pi 1516) in Egypt; cf. Barrington Atlas map 70 grid C3).
[2] Greek Anthology 6.332.1-3 (Hadrian). Hadrian composed the dedication at Antioch (alpha 2692, on the south bank of the Orontes; cf. OCD(4) s.v. Antioch(1)) in 114 CE. Trajan delivered the dedication at Zeus's temple on Mount Kasios and fortified it with spoils from the earlier victories over the Getae (Dacians; cf. delta 20 and OCD(4) s.v. Dacia); cf. Birley, p. 64. On this epigram, see Page (561-563) and its further extracts at epsilon 1586, kappa 2391, lambda 609, and omicron 955.
[3] From alpha 1510: see note 2 there.
[4] Greek Anthology 5.217.1-2 (Paul the Silentiary); cf. alpha 1510 and kappa 2075. See further excerpts from this epigram at delta 57, epsilon 919, and rho 317.