It doesn't appear that Harbinger even had a fifth leg. There's no place for it in Harbinger's body. There's no arch in the front of Harbinger where the fifth leg would attach to. Where Harbinger has the lower four eyes, there should be an arch and an indent for the fifth leg to attach to. There isn't. Harbinger most likely had four legs to begin with instead of five.
I noticed harbinger moves around quickly on ground ( I know in ME3 he just stood there shooting at you but have you noticed he moved his body around faster than the other reapers) im guessing if a reaper has les legs it can walk around faster like harbinger