From Diablo Wiki
 Uber Act Bosses
These Uber Monsters can be found in Uber Tristram:
 Summoned Monsters
Uber Tristram is also full of different summoned monsters that are also dangerous for those who do not take their existence into consideration.
Some of the monster that are includes are:
Note: These monster are very strong compared to any of their counterparts, appear randomly across the map, and all of them have immunities.