What does the Greek suffix "-sus" mean?

well for starters "us" is a suffix used solely on Roman names, or Greek names which the Romans latinised when writing down, so Dionysus would have been Dionysos and Pegasus would have been Pegasos. To be honest i don't think you'll have much look finding a meaning for "sos" on the end of a name, its like meeting someone called James and asking what "mes" means, its just an ending commonly used on names, and its not used excessively, you say there are many many more of them, but looking through a list of Greek deities it seems no more a common ending to a name than any other, it just quite a famous one.