I found this colorized version of Charlie Chaplins "Shanghaied" from 1915.
https://www.youtube.com/watch?v=Bp-SnrvqRrs
In the first 6 minutes there is one man (actully one more a short time) visible almost all the time, wearing black rainwear trousers. I guess it was some sort of rubber. I don't think plastic was invented back then. Is this the oldest example of rain trousers in movies?