The Infero alternate upgrades appear to have escaped from the freak show where they were held captive. You can see them by following these links:

• Quasit: Siphon Mana
  
• Horned Leaper: Leap, Enraged
  
• Firebreather: Three-headed Attack, Fire Breath. No Enemy Retaliation. Fire Breath
  
• Succubus Temptress: Shooter, Ranged Retaliate, Immune to Hypnotize (aka Puppet Master), Seduce
  
• Hellmare: Fear, Searing Aura, Large Creature
  
• Pit Spawn: Axe of Slaughter, Magic-Proof (50%), Large Creature
  
• Arch Demon: Flyer (Teleportation), Summon Other, Large Creature

Edit: The stats of the units can now be found on the official forum. Thanks to evilp who found the links first.