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?

The trouser fabric may be waxed cotton.

Yes, it may be, but the trousers seems very soft in the movie. Waxed cotton is stiffer, isn't it?

Charles Macintosh sold his first rubberised clothes in 1824 (wikipedia).