The thematic derivation would express association with a root noun *h₁eḱ- (“swiftness, celerity”), thus denoting "that which has swiftness".
According to Kloekhorst (2008), however, the original derivation was an u-stem, to which point Anatolian reflexes which presume Proto-Anatolian u-stem *h₁éḱu-, as opposed to the thematic (o-stem) derivation in all the other PIE branches. There is no known phonological development through which PIE *h₁eḱwo- could yield PAnat. *h₁eḱu-, and in view of the productivity of the o-stem inflection in Anatolian it is unlikely that PIE *h₁eḱwo- would have yielded PAnat. *h₁éḱu- through secondary developments. We therefore must conclude that the PAnat. u-stem *h₁éḱu- reflects the original state of affairs and that the thematicization as visible in the non-Anatolian IE languages (which is a trivial development) must be regarded as a common innovation of them. In other words, this is one of the evidences supportive of the Indo-Hittite hypothesis.
The original paradigm was probably thus *h₁éḱ-u-s, *h₁éḱ-u-m, *h₁ḱ-u-és, from the stem *h₁eḱ-u- (“swift”).
Another possibility, by bringing Proto-Altaic *èk`á (“to paw, hit with hooves; to move quickly, to rage”) into consideration, apparently brings more light into the original meaning of the Proto-Indo-European word for ‘horse’. The behavior of wild horses seem to be indicated in the Altaic terms, as in Written Mongolian aɣsur- (“to storm, to fly into a rage, to beviolent or furious; to be fiery”), aɣsum (“fury, rage, madness; furious, fiery, violent, tempestuous, spirited”), aɣsum mori (“fiery or spirited horse”), Khalkha agsčix (“to be fiery, chafe, behave nervously (of a horse); to continually rage”), or Kalmyk agsag (“wild”).
Thus, by examining the supplementary Altaic material, the original meaning of Proto-Indo-European *h₁eḱ-u- becomes clear. It may not have meant ‘the swift one’ but, rather, ‘the spirited, violent, fiery, or wild one’. But this circumstance could not have been seen on the basis of the Indo-European evidence alone.
For both, the Proto-Altaic and the Proto-Indo-European forms, a tentative hypothetical Proto-Nostratic root *ʔekh₁- ("to move quickly, to rage; to be furious, raging, violent, spirited, fiery, wild") is proposed.
- Anatolian: *ʔeḱu-
- Hittite: 𒀲𒆳𒊏𒍑 (*ekkus) [ANŠE.KUR.RA-us] (only attested with Sumerogram)
- Cuneiform Luwian: *aššu-/*azzu- [ANŠE.KUR.RA-us]
- Hieroglyphic Luwian: EQUUSásù(wa)
- Lycian: esbe
- Celtic: *ekwos
- Germanic: *ehwaz
- Avestan: 𐬀𐬯𐬞𐬀 (aspa)
- Baluchi: اسپ (asp)
- Laki: اسب (asb)
- Median: *aspa-
- Old Persian: 𐎠𐎿 (asa)
- Pashto: آس (ās)
- Sogdian: ʾsp (əsp, asp)
- Kamviri: ušpa
- Italic: *ekwos
- Phrygian: es'
- Thracian: esvas
- Tocharian: *yä́kwë
- Julius Pokorny (1959), Indogermanisches etymologisches Wörterbuch, in 3 vols, Bern, München: Francke Verlag, volume I, page 301
- Alwin Kloekhorst (2008), Etymological Dictionary of the Hittite Inherited Lexicon, Leiden, Boston: Brill Academic Publishers, page 10
- J. P. Mallory, D. Q. Adams (eds.) (1997), Encyclopedia of Indo-European culture, London: Fitzroy Dearborn Publishers, page 273
- Don Ringe, From Proto-Indo-European to Proto-Germanic, Oxford University Press, 2006
- Allan R. Bomhard, Proto-Indo-European ‘Horse’ From a Nostratic Perspective, Charleston, SC, USA (PDF, edu)
- Sergei Starostin, Vladimir Dybo, Oleg Mudrak (2003), Etymological Dictionary of the Altaic Languages, Leiden: Brill Academic Publishers: “*èk`á”