Buff Viaton is what happens to Viaton whenever he absorbs someone of significant strength (comparatively). He has shown multiple buff forms, and each one seems to slow him down.
MusclelyThis version has some muscles, but not enough to be in his way. This happened when he absorbed Edgelord, and he easily overpowered Zeno with it.
With Ultra Rage, Viaton's muscles should in theory increase dramatically, especially with the uncontrolled power of Rotu-4 absorbed. Unfortunately, he vanished into Universe 6 before he finished transforming.