Shadar-kai (Gloom Weaver, Shadow Dancer, Soul Monger)
Elemental Myrmidons (reprint from Princes of the Apocalypse)
Demon Princes (Baphomet mentioned, Zuggtmoy and Jubilex hinted at)
Confirmed as six chapters; The Blood War, Elves, Dwarves & Duergar, Gith And Their Endless War, Halflings And Gnomes, and Bestiary.
One shot contains a Races bar featuring Dwarf, Elf, Gith, Gnome and Tiefling. It's implied we'll be getting reprints of the Duergar and Svirfneblin from SCAG, alongside the new Eladrin & Shadar-kai they've been promising, as well as the Gith and the Archdevil Tieflings.
Also, it seems like Gith will basially be a complete reprint of their UA material; the teaser shot of the Gith Characters page mentions both races get +1 Int and the snippet of Gith info features a Strength boost and the Decadent Mastery feature.
Duergar Soulblades and Xarrorn (both from OotA), as well as Duergar Hammerers and Screamers (which aren't from OotA, unless they are renamed variants).
Ogre Battering Rams and Ogre Bolt Launchers (which sound like ogre siege equipment, but they are listed under creatures, so, presumably, living ogre siege equipment)
"Merrenoloth", which I assume is either a misspelled, or a new spelling for, Marraenoloth, although its CR 3 is much lower than the CR 10 it had in 3.x days.
Iron Cobra and Meazel are both mentioned in the Bestiary chapter introduction (as is one more creature who would appear after the "and..." but is cut off at the bottom of the screen).
I'm very happy to see more ogre and troll variants, which will spice up encounters with that type of creature.
The Winter Eladrin is interesting. We know that the playable eladrin will have seasonal variants (even within the same character), but we had assumed that the creature eladrin would revert to the pre-4e versions. I'm guessing that they've reskinned some of the pre-4e versions as eladrin with seasonal names (although they could have scrapped the old versions entirely and relaunched brand new types, but that seems to be against the MO of 5th Edition). So, if that's the case, which of the older types is our new CR 10 Winter Eladrin? The CR 10 eladrin back in 3.x was the Firre, but as it's fire based, it would be utterly inappropriate for a Winter Eladrin (and would easily be the Summer Eldadrin). My guess would be the Bralani (although CR 10 is a bit of an upgrade for them), as they traditionally inhabit Pelion (the desert and snow filled layer of Arborea), were able to turn into a snowstorm (among other forms), and have snow white or silver hair.
(Also, this preview has checked off even more from my "100 most wanted monsters" list. Between Volo's and this, I might see the huge majority of them updated for the current edition!)