Same with Lohan. Lohan is a catch all term. Northern, Southern, qigong, set names, etc... I don't think there's an actual "Lohan Quan" per se. I think it's more of "Hey ,whadda we call this? It needs a cool name people with remember!!" type thing... :O
There is a Lohan/Arhat Chuan but not many places teach the complete system. I think originally there were 10 forms or so but 5 of them were taught at Jing Mo. I only learned the first form... it's similar to Bak Sil Lum but a little more "springy". At least in the one form I learned.