The Stanwick Horse Mask is on show at the National Museum of Scotland in the Celts exhibition. I saw it at the British Museum and settled on it as a subject for a test painting. I need a lot of practice ... I am happy enough with it but only so far. More practice definitely required!
Note that this is not really a "mask" but an ornament attached to some sort of container or bucket, perhaps for ale. It is not very big (only about 10cm tall) but has a cartoon quality, well expressing the horse with an economy of style in the copper alloy. Probably from about 50 AD and dug up in Stanwick, North Yorkshire, in 1843. Details.