OK, so:

Axe: Grom Hellscream
Warlock: Orc Warlock
Disruptor: Thrall
Earthshaker: Tauren unit
Elder Titan: Tauren Hero unit
Treant Protector: that Night Elf tree unit/building
Tiny: Mountain Giant
Batrider: Troll Bat Rider unit
Witch Doctor: Troll Witch Doctor unit
Troll Warlord: Troll Berserker unit
Venomancer: Hydralisk
Beastmaster: Rexxar (It was right on the tip of my tongue!)
Brewmaster: Pandaren Brewmaster
Ursa: Furbolg neutral creep
Lone Druid: Druid of the Claw Night Elf unit
Lycan: Druid of the Fang Night Elf unit
Gyrocopter: Dwarven Gyrocopter unit
Sniper: Dwarven gunner unit
Tinker: Goblin Tinker unit
Alchemist: Goblin Alchemist unit
Timbersaw: Goblin Forester unit
Clockwerk: Dwarven Mortar Cannon unit
Techies: Goblin Sapper unit
Kunkka: Admiral Proudmoore
Keeper of the Light: Archmagus
Shadow Shaman: Orc Shaman unit
Ogre Magi: Ogre Magi unit
Nature's Prophet: Malfurion Stormrage
Enchantress: Night Elf Dryad unit
Centaur Warrunner: Centaur neutral creep
Naga Siren: Naga Siren unit
Slardar: Naga soldier unit
Slark: Murlock neutral creep
Tidehunter: Ocean Giant
Omniknight: Paladin hero unit
Broodmother: Crypt Fiend Scourge unit Giant Spider neutral creep (And here's my first stumble)
Nyx Assassin: Crypt Lord Scourge unit
Sand King: Giant Scorpion neutral creep
Morphling: Water elemental unit
Io: Night Elf Wisp Worker Unit
Mirana: Malfurion's girlfriend, the Night Elf High Priestess warrior lady.
Luna: The basic Night Elf unit Malfurion's girlfriend was the souped-up Hero unit version of.
Terrorblade: Illidan Stormrage
Skeleton King: King Leoric from Diablo
Doom: Diablo Doom Guard (I don't even REMEMBER that guy.)
Dark Seer: Either an Eredar hero unit or a Protoss High Templar Draenei Fallen One (I was close)
Juggernaut: Orc Blademaster hero unit
Chaos Knight: Death Knight Custom Model (HOW WAS I SUPPOSED TO GUESS THAT?)
Dragon Knight: uhhhhhhhhhhhh... Human Footman unit
Sven: Orc Grunt Overlord Felguard (OK now I see it, but why'd the basic Human melee unit get in and the Orc one didn't?)
Chen: High Elf Priest unit Custom Model (NOBODY EVER SAID DOTA HAD CUSTOM MODELS IN IT, HOW WAS I SUPPOSED TO KNOW?)
Invoker: Expansion's Blood Elf hero guy
Legion Commander: That Alliance bastard who drove Blood Elf hero guy into the arms of the Naga
Lifestealer: Scourge Ghoul unit
Pudge: Scourge Abomination unit
Bristleback: Quillboar neutral creep
Magnus: Wooly Mammoth Centaur neutral creep
Tusk: Walrusman neutral creep? [s]Was that even in WC3?[/s] Apparently it was.
Riki: Satyr neutral creep
Enigma: Protoss Archon Custom Model (GAH!)
Necrophos: Scourge Necromancer unit Soul Keeper Ghost (... Who?)
Lich: Scourge Lich unit
Earth Spirit: Brewmaster's special attack
Storm Spirit: Brewmaster's special attack
Ember Spirit: Brewmaster's special attack
Clinkz: Scourge Skeleton unit Burning Archer (I have no recollection of this guy.)
Meepo: Kobold neutral creep
Drow Ranger: Sylvanas Windrunner AFTER she got her body back
Windranger: Sylvanas Windrunner BEFORE she lost her body
Bane Elemental: Scourge Phantom unit Voidwalker (Wow, he lost weight!)
Queen of Pain: Succubus unit
Phoenix: Phoenix neutral creep Archmage Ultimate Summon (OK, so maybe I didn't play as much WC3 multiplayer as I should have...)
Medusa: some neutral creep Lady Vashj (Yeah, I have no excuse for missing this one. It's gonna turn into one of those things that you suddenly remember and now you can't go to sleep.)
Ancient Apparition: Frost Elemental Custom Model (I didn't know you could import stuff into WC3.)
Outland Devourer: ...something from the expansion pack, I know that much. Obsidian Destroyer Undead Unit (There was a thing in WC3 called that? Really?)
Visage: Scourge Gargoyle unit Ghost Wyvern Orc Unit (I remember the Wyverns, but I don't remember any of them being ghosts.)
Zeus: ...Yeah I have no clue where this guy came from. Mountain King hero unit (I was wondering what had happened to that guy. Did he REALLY have nothing but lightning abilities? I thought he was more varied.)
Phantom Lancer: Chinese Monkey King, but only because somebody posted it here. Final Fantasy X import. (Eew.)
Skywrath Mage: ...Blood Elf unit from the expansion, right? Dragon Rider
Anti-Mage: ...ALSO a Blood Elf unit from the expansion? Illidan Stormrage pre-corruption. (I'm used to thinking of him AFTER he was corrupted, OK?)
Leshrac: ...Cenarius? (Seriously? I got that one right? I just pulled it out of my @$$)
Bounty Hunter: uh, some unit from the Mercenary Taverns? Custom Model (Ugh.)
Templar Assassin: Night Elf unit Warden
Crystal Maiden: Alliance Sorceress unit Jaina Proudmoore
Lina: Jaina Proudmoore? Alliance Sorceress unit (I was going to rant about how easy it could be to confuse the two, but then I remembered that Jaina walks while the Sorceress floats, so instead I'll just sit here and stew in my own failure.)
Abaddon: Death Knight Arthas
Shadow Fiend: Tichondrius Firelord (What is that, a boss level Fire Elemental? I don't remember this one.)
Shadow Demon: Generic Dreadlord unit Eredar (I was close)
Death Prophet: Scourge Banshee unit
Night Stalker: ALSO Scourge Gargoyle unit? Dreadlord (Oh, he's the Dread Lord...)
Undying: ...Definitely Scourge... Custom Model (*thud* *thud* *thud*...)
Vengeful Spirit: Hero version of Scourge Banshee unit Night Elf Warden Ultimate 'Avatar of Vengeance'
Bloodseeker: Neutral creep of some sort Orc Shaman (Wait, they used Orc Shaman twice?)
Viper: Neutral creep of some sort Nether Dragon (Nether Dragon?)
Huskar: Troll headhunter?
Dazzle: Gnoll/Scourge Cultist unit Dark Troll Shadow Priest (I should have known this one.)
Pugna: Kel'Thuzad Skeleton Mage Necromancer summon (They could summon these? I don't remember that.)
Spectre: Maybe THIS is the hero based on the Scourge Phantom unit? Maybe Bane is from Starcraft? Similar to Avatar of Vengeance (What does that even mean?)
Puck: I got nothing. Night Elf Faery Dragon unit (That was a thing?)
Jakiro: Dragon Whelp Neutral Creep? Night Elf Two Headed Dragon unit (This was added in the expansion pack, right?)
Spirit Breaker: He's that catguy, right? I have no clue where he's from Orc Spirit Walker unit (Not ringing a bell)
Weaver: Zerg unit Crypt Fiend (I should'a guessed, he has the exact same body plan and everything!)
Phantom Assassin: Scourge Night Elf Warden, Hero version (*slap*)
Silencer: Expansion Blood Elf Spellbreaker unit (That was like betting on [i]red[/r] on a Roulette table)
Lion: Burning Legion Modified Archimonde model
Rubick: No idea. Custom Model (Should I even be counting these?)
Razor: Also Burning Legion? Revenant (D'OH!)
Faceless Void: Demon/Expansion Faceless One (Oh yeah, I remember those guys now!)