https://www.reddit.com/r/darksouls3/comments/486qtr/spoiler_a_few_bosses/
As seen from the soundtrack CD. Not all of the bosses.
Iudex Gundyr
Volde, of the Frigid Valley
Curse-Rotted (Ridden?) Greatwood (seen in trailer)
Crystal Sages
Dragons of the Deep
Dancer of the Frigid Valley (seen in the network test)
Old Demon King
Ozroes(?), the Condemned King
Ancient Wyvern
Nameless King
Abyss Watchers
Yhorm, the Giant
Aldrich, Devourer of Gods
Lothric Elder Prince, Lothric Younger Prince
Soul of Cinder