I own a 2020 Trek Farley 9.6 Carbon Fat Bike with the MASTODON PRO EXT fork on it, works great, comes set at 100mm but you can adjust to 120mm or 140mm depending on your riding. The fork is light for it's size! Smooth travel and the lock out is nice but you will have to run a 180mm rotor on it so you might have to upgrade your standard 160mm to a 180mm.
Bought my Specialized Fatboy a year ago with carbon fork. I noticed in my area of SE Michigan that the rock gardens, roots and logs were really taxing me on long and fast descents. While the fatty tires absorb slot of the vibrations and shock it still is not like having a quality shock out front. Took advantage of Hayes sale and purchased Mastodon Pro. I have been blown away by the improvement not only in my descents but the handling on sharp twisty single track and my confidence as a result. Customer service is spot on as well. Once I got it dialed in, it’s a set it and forget it fork for my fatty! You won’t be disappointed!