You can read the full list of new alien types, below. Some of the alien enemies it adds are unique, while others are twists on existing XCOM 2 units.
You can find the Alien Pack on the Steam Workshop (note that it updates and replaces the ‘Muton Centurion’ mod). Variety is the spice of life, so why not introduce a few more horrifying alien threats into your XCOM 2 sessions? That’s possible today, thanks to the latest in a series of free mods created by the ‘Long War Studios’ chaps: the Alien Pack.