Vergil tells us:
ductores Danaum … montis equum diuina Palladis arte/aedificant, sectaque intexunt abiete costas ['The Danaan chiefs … build by Pallas’ divine art a horse of mountainous bulk, and interweave its ribs with planks of fir (abies: ‘the silver fir’)']. [Aeneid, 2:16]
But wait a minute:
... praecipue cum iam hic trabibus contextus acernis/staret equus ... ['… when yonder horse now stood framed of maple-beams (acernus) …'] [2:112]
And here again:
...roboribus textis caeloque educere iussit ... ['… this mass of interlaced oak timbers (robor) so huge …'] [2:186]
And once more:
inclusos utero Danaos et pinea furtim/laxat claustra Sinon ... ['Sinon … stealthily sets free from the barriers of pine [pineus] the Danaans shut within the womb…'] [2:258]
Of line 112, there, T E Page [The Aeneid of Vergil ed. Page (London 1902; 2 vols) 1:216-7] says: ‘In 16 Virgil talks of “planks of pine”, here of “beams of maple”, and 186 of ‘woven oak-timbers.” Sigdwick calls this “a natural poetic variation”: as a matter of fact it is a curious illustration of Virgil’s art. He prefers the particular to the general and therefore prefers to name some particular tree rather than to speak simply of wood.’
Preferring the particular to the general is all very well and good; but it's not too much to request he prefer the same particular. (Page goes on: ‘[he] is consequently led to this artificial and unnatural method of giving three different names to the same wood. The difficulty he labours under in endeavouring to lend a poetical character to his description of the horse is also shown by his using the same metaphor (intexunt, contextus, textis [‘woven’]) in all three passages.’) Also it seems to me that there's no warrant for assuming, as Page does here, that 'fir' is the same thing as 'pine'.
What we have, then, is a horse made of: silver fir [abies]; maple [acernus]; oak [robor] and pine [pineus]. Perhaps, Page's dismissal notwithstanding, Vergil's repeated use of the idiom of weaving means he wants us to think of the horse as actually made of four different types of wood, all platted together? That's not outside the kingdom of possibility, I think; although it strains credulity (for if that was what Vergil meant, then wouldn't he say something along those lines? 'Woven severally of many timbers', or something?)
Silver Fir was well-known to the Romans. It is common in northern Europe and grows as far south as southern Italy and as far easy as the Carpathians; though not in Asia Minor: so it doesn’t seem likely the Trojans would have supplies handy outside Troy to fashion their horse. (Wikipedia also says: ‘Silver Fir is the species first used as a Christman tree.’) Most of the 125 species of Maple are native to Asia, and some to Asia Minor, so that’s a more plausible wood: though it would be difficult to ‘weave’, being a particularly hard hard wood (Wikipedia: ‘it is the wood of choice for bowling pins, bowling alley lanes, pool cue shafts… [and] is also used for the production of wooden baseball bats, though less often than ash or hickory due to the tendency of maple bats to shatter when broken.’). Oak grows widely in northern Europe and north America (I can’t find out whether it grows in Asia Minor) and makes good ships and furniture. It’s an important symbolic tree: the tree of Thor, the thunder god, in Norse Mythology; and the sacred tree of Zeus in Greek (the oracle of Dodona in prehistory consisted solely of a holy oak.) Pine provides ‘timber that is denser, more resinous, and therefore more durable than spruce (Picea). Pine wood is widely used in high-value carpentry items such as furniture, window frames, paneling and floors.’
What can we make of this? Of course, perhaps it is simply carelessness on Vergil’s part. But let’s, for the fun of it, imagine otherwise: imagine that there’s a significance here. Beyond straining to fit these terms into improbable dactyllic hexameters:
Sic: Abies acernus [duri] robor[is] pineus
— u u — u u — u u — u u — —
(But shouldn’t the ‘e’ in ‘acernus’ be long, because it comes before two consonants? Or does ‘rn’ not ‘make’ position?) … as I say, beyond such pointlessness, what of the trees themselves? For trees are symbolic of supernatural forces. The fir, for example, stands for ‘Science or Knowledge’ (to quote Graves’s White Goddess: I have no intention of trying to justify the authority of this source): ‘a form of the Greek Elate (‘fir tree’); Elatos (‘fir-man’) was an early Achaean King of Cyllene … [the fir] may thus be equated with Osiris, or Adonis, or Dionysis, who was born from a fir and mothered by the horned Moon-goddess’ .
Oak is Zeus’s tree, of course; the largest in the forest. Of pine-trees, Graves quoted Câd Goddeu, ‘The Battle of the Trees’: ‘The pine tree in the court/Strong in battle,/By me greatly exalted/In the presence of kings’: Ares’ tree, in other words. Maple is harder to pin down, although the Latin name is derived from ‘acer’, meaning ‘sharp’ (because of the shape of the Maple’s leaves, perhaps) but also meaning ‘bitter, harsh, poisonous’. In other words, Vergil constellates his constructed horse, at Troy, as a trinity of Achaean gods: Zeus, Ares and Dionysis—signifying that the horse will bring dominion over Troy via war and frenzy—together with the bitterness the horse bodes for all Trojans.