I've read many explanations about how recent history/popular depictions of either vikings, or anglo-saxons wearing helmets with horns are incorrect. That it is a disadvantage if a blow is taken to the helmet, it was invented by Wagner, etc. However, in the recent video game Ghost of Tsushima, set during the Mongol invasions of Japan, the main character's helmet features a prominent set of stag antlers. I remembered also this is mentioned very briefly in James Clavell's novel Shogun, set just prior to the Togugawa Shogunate, that a character is wearing a horned helmet. Checking the wikipedia, there look to be several surviving examples of horned helms from the Edo period. So what's the deal here? Were these ceremonial, or are there just no surviving European examples?
I'm assuming you saw maybe the horned helmet of Kuroda Nagamasa or Honda Tadakatsu (although horns were amongst the favourites for sengoku warriors).
Japanese helmets began seeing wilder variations during the mid-late Sengoku period because the samurais' need for noticeability in increasingly larger warfares. Plus, many decorations kinda served as charms that samurais used to pray for good luck (for battlefield protection and personal wealth). Thus, the purpose of horns, antlers, or even big spikes were really not about protection.
There were both ceremonial and practical examples - although sometimes it's difficult to assess which purpose a specific helmet served. Plus, fancy helmets gradually became a symbol of identification for daimyos under Hideyoshi. For example, Maeda Toshiie's famous helmet had both a display version (much larger) and a battlefield version (smaller). As we move onto the Edo period, battles became less and less frequent - and their ceremonial function were a lot more prominent.
I do not believe helmets had horns during the Mongol invasion (I know a lot less about pre-Sengoku history) - as there was no need to. However, horned helmets were used in battlefields - not just for ceremonial purposes.