In fact, since the sensible way to do this is MST-style, I'm going to invite Agent Huinesoron to cover this one for me. ^^
The deer were a truly ancient folk.
True. We predate humanity by millennia.
Thousands of years before ponies could write or speak,
Or even existed.
the various deer species
Clearly an analogue for the Three Tribes, though 'species' implies we couldn't interbreed - which would make me a biological impossibility.
had filled much of the world with civilization, forging great kingdoms
Now that isn't true. Before humanity awoke, we were all living in Valinor; most of the world was empty. No, actually, I take that back: the Avari and the Sindar were still there, and I suppose their lands count as 'civilisation'. Just about.
and building magnificent cities of crystal and precious gems,
That's mostly true in Valinor. Unless you count caves with those things set into the walls, in which case Menegroth probably counts as well. (And Nargothrond, naturally, but we built that after the coming of the Sun.)
their shared culture fueled by their strange and powerful magics.
Now that's just nonsense. I mean, we had our arts, sure, but 'magic'? No. Working a mail-coat so that it doesn't catch the light isn't sorcery - it's just a skill.
Unlike ponydom, where complex magical skill was restricted to unicorns,
Or humanity, who are frankly rubbish at anything they'd call 'magic'; look at the Numenoreans, who tried to rip off our metalwork. Fingolfin's sword wounded Morgoth seven times, while a Numenorean knife dissolved after striking the Witch-King just once. Pathetic.
every deer could use magic;
Every elf could learn certain skills, yes. That's how skills work.
moreover, the average deer was far more powerful than the average unicorn, their pair of many-tined antlers able to channel and conduct much more magical energy than a unicorn's single horn.
Uh. Only if you interpret 'antlers' as a metaphor for our very nature.
(hS note: but I do love that idea. It's logical!)
Because of this, they had not left many technological achievements behind,
Nonsense! Swords, stonework, the great shipworks of the Grey Havens - it's not our fault mortals wander on the outskirts of Mirkwood or Lorien and go 'oh, all elves live in trees, eating leaves and drinking dew'.
save their unparalleled ability to shape crystal and gemstone.
Reference to Feanor, or to Celebrimbor? Either way, that only really applies to the Noldor; the other tribes either ignored such things (the Vanyar), or preferred to leave them in natural form (the Teleri).
Their cultural achievements were great, however: the deerfolk's languages had formed the foundations of many modern languages, not just in Equestria but across the world.
That's true. The main exception is the Dwarven tongue - and I suppose the rumoured original tongue of mortals, though I don't believe it ever existed.
Much of common law
We did shape a lot of that. The Edain were pretty primitive, legally speaking, when they reached Beleriand; things got much better when they started adopting our laws.
and accepted interspecies customs had been laid down first by the deerfolk.
I'm not sure what that means. That we didn't kill the dwarves and mortals when we first met them, so they didn't kill each other either? I guess. We're pretty cool that way.
Their artistic achievements were magnificent, ancient deer art and sculpture renowned for its elegance.
^^ Why thank you.
Deer had also invented the performing arts, with a particular love for the theater.…
Is that a backhanded complement? It feels like one, but I'm not sure how. Either way, technically 'the performing arts' were invented by Vana and Nessa when they invented dancing, and dance and music were always our favourites; though if you don't know about the Valar, I suppose you could attribute it to us.
Yet taken together, these were but echoes of the past, and faint ones at that. There was much about the ancient deer that most ponies did not know,
You should hear some of the nonsense they said about us. 'So there is a witch in the Golden Wood' - about the Lady Artanis! Really, I ask you!
due largely to the one thing everypony did know about them. One day, for reasons nopony fully understood, the deer had gone to war with each other.
What? No, we didn't.
It had been a terrible conflict, spreading across the Earth,
No it didn't!
lasting decade after decade,
Like that's even a long time - but no, it didn't!
unleashing ever more powerful destructive magic.
No, we didn't! Why would we do that?
When it was over, the whole world had lain in ruins,
Now that... okay, I think I see where this is coming from. This story has conflated us with the Valar - and included Morgoth in that number. So yes, I suppose 'the pre-human people' went to war with 'themselves' and laid a large portion of the world to ruin with terrible forces - but that was us versus Morgoth, not a mass Kinslaying.
Though... I suppose there's the argument that orcs are elves, and that we were technically all kinslayers all along... but somehow I don't think that's what they're going for.
and most of the deer had wiped themselves out.
Unfair! We were slaughtered by Morgoth, and later Sauron, not by 'ourselves'. Well, I guess the disaster of the Nirnaeth could count as suicide - but not the rest of it!
Those who had survived had withdrawn deep into the world's forests, and in this age they were rarely seen.
What, the vast forests of Rivendell? The deep woods of the Grey Havens? The reclusive massive trade network of Mirkwood?
Yes, we 'withdrew' - in the same way that mortals 'withdrew'. When the world is filled with orcs, trolls, spiders and other horrors, you don't go larking about in open countryside; you build walled cities, or deep caverns, or hidden refuges. The cities of Gondor had high walls; the people of the Shire never left their country, and were guarded by the Dunedain; and we tended to stay in our own settlements. But not 'deep in the forests'; neither of the two forest realms were particularly 'deep'.
In general, it's clearly inspired by our history - but I think it's more inspired by general D&D-type elves. 'Ooooh, powerful magic & no technology & destructive war & now they all live in trees' - it's us in the broadest outlines, but the details are from somewhere else.
I think he's regressing a bit - he's not meant to be that condescending any more. ^_~
hS