Per todays D&D podcast, Chris Perkins confirms Shadow Mastiffs are in Volos Guide.
Good to hear!
I assume he didn't drop any other hints towards the contents?
Perkins on Twitter this weekend says that he likes what they have done with Hags in Volos Guide: so some sort of extended Hag variants?
Spellweavers
Any monster, just as long as they are significantly more dangerous for their CR, with more tools in their toolbox to combat trivial tactics to prevent them from closing to melee and other issues.Which monsters do you want to see?
Mindflayers in todays podcast "Lore You Should Know," confirms there will be a map and detailed description of a Mindflayer colony, discusses variants, etc. Mostly fluff discussion, but chapter one will have dungeon maps.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.