Older feminine form of Bhādrapadā, expressly equated with it. Colebrooke's gloss 'The wing of Pegasus' fixes the asterism on the Pegasus square. The archaic form underlies Kauṭilya's month-name Prauṣṭhapada (Arthaśāstra 2.20.55), confirming it was still standard when the Arthaśāstra's calendar was compiled; the Atharvaveda Nakṣatra Kalpa also uses Proṣṭhapadā rather than Bhādrapadā.
प्रोष्ठपदा Proṣṭhapadā
Markab / Algenib · α and γ Pegasi · Pegasus
Attestations
Transcribed from the Devanagari OCR of the printed edition at shloka_source_url (Achyuta Nand Jha ed., vol. 1) and checked word-for-word against two further independent scans of two different printed editions of the same commentary — Krishna Chandra Dvivedi's (https://archive.org/download/brihatsamhitawithbhattotpalavivrittipartidr.krishnachandradvivedi/Brihat%20Samhita%20%20with%20Bhattotpala%20Vivritti%20Part%20I%20-%20Dr.%20Krishna%20Chandra%20Dvivedi_djvu.txt) and A.V. Tripathi's (https://archive.org/download/JiUm_brihat-samhita-part-1-of-varaha-mihira-by-avadh-bihari-tripathi-saraswati-bhavan-granthamala/Brihat%20Samhita%20Part%201%20of%20Varaha%20Mihira%20by%20Avadh%20Bihari%20Tripathi%20-%20Saraswati%20Bhavan%20Granthamala_djvu.txt). Only glyph-level OCR artifacts that all three scans show to be artifacts (e.g. OCR रद्वव for रद्गव, श्व for श्च) have been normalised; every substantive reading is as scanned, and the two places where the scans genuinely differ are flagged in identification_notes.
'The earlier Proṣṭhapadā' = PŪRVA-BHĀDRAPADĀ, in the archaic Proṣṭhapadā form the database already records from the Amarakośa. Here it is carried by a pre-Varāhamihira authority: Parāśara's vīthī scheme quoted by Utpala on BS 9.4, 'govīthī prākproṣṭhapadādīni catvāri' — the Go-vīthī is the four beginning with Prākproṣṭhapadā. That is the same four-asterism Go-vīthī (Pūrva-Bhādrapadā, Uttara-Bhādrapadā, Revatī, Aśvinī) that Varāhamihira gives at BS 9.2 and the Samāsa-saṃhitā counts 'from Ekapāda', which cross-confirms both name-equations.
Devanāgarī from Sanskrit Wikisource, which prints the sūtra with an analytic hyphen. GRETIL's two IAST texts give 'phalgunī-proṣṭhapadānāṃ ca nakṣatre'. The related sūtras read: 4.2.35 महाराजप्रोष्ठपदात् ठञ् (GRETIL: mahārāja-proṣṭhapadāṭ ṭhañ) and 7.3.18 जे प्रोष्ठपदाणां (a Wikisource typo for प्रोष्ठपदानाम्; GRETIL correctly: je proṣṭhapadānām).
The archaic name of the Bhādrapadā pair, and Pāṇini's only name for it — Bhādrapadā occurs nowhere in the Aṣṭādhyāyī. Pāṇini names Proṣṭhapadā in three separate sūtras of his own: 1.2.60 (paired with Phalgunī, and expressly marked nakṣatre, 'when it denotes an asterism'), 4.2.35 mahārāja-proṣṭhapadāṭ ṭhañ (deriving the month-name), and 7.3.18 je proṣṭhapadānām. It also appears at 5.4.120 in a list of bahuvrīhi finals, but there proṣṭhapada is probably the ordinary 'having feet like a bench/carp' from which the asterism name derives, so I do not count it. Patañjali comments on 1.2.60 with the forms 'udite pūrve proṣṭhapade, uditāḥ pūrvāḥ proṣṭhapadāḥ' ('the earlier two Proṣṭhapadās having risen' / 'the earlier Proṣṭhapadās having risen') — showing the asterism was already understood as a PAIR (pūrva/uttara) in the 2nd c. BCE — and on 7.3.10 (his numbering; = 7.3.18) with 'proṣṭhapadāsu jātaḥ proṣṭhapādaḥ brāhmaṇaḥ', 'a brahmin born under the Proṣṭhapadās'. That 1.2.60 is a rule about DUAL versus PLURAL for Phalgunī and Proṣṭhapadā is itself an astronomical datum: Pāṇini knew each as a group of stars that could be counted either way.
CE reference R_1,017.010 = vulgate Bāla Kāṇḍa 18.16. Unlike the birth-horoscope verse two ślokas earlier, this one IS in the Critical Edition.
Two useful data. (a) Rāmāyaṇa 1.17.10 (Critical Edition — this one IS in the CE) compares Daśaratha's four sons to Proṣṭhapadā: 'handsome and worthy, in radiance LIKE THE PROṢṬHAPADĀS'. The point of the simile is number: four princes, four stars. Griffith footnotes 'One of the constellations, containing stars in the wing of Pegasus.' This four-star reading of the combined Pūrva- plus Uttara-Proṣṭhapadā (α, β, γ Pegasi + α Andromedae) is exactly the Pegasus square and is an early independent witness to it. (b) MBh 6.3.14 uses the pair form 'proṣṭhapade pūrve' / 'uttare' for Venus's transit, showing the eastern/western split already fixed. The database carries Proṣṭhapadā from the lexicons; these are earlier.