So, in WW1, in late 1915. Serbian government decided to retreat across mountains over Montenegro and Albania to Greece (the Great Retreat) https://en.wikipedia.org/wiki/Great_Retreat_(Serbian). Probably most of you heard for this.
There is story that one Serbian soldier carried on his back hill cannon (this is literal translation, I don't know exact name of weapon). In harsh winter mountain condition just surviving is great achievement, and carrying cannon on your back in those conditions is something unimaginable. Soldier's name was Jevto Jevtović. According to story, when Serbian army arrived in Greece, French general Guillaumat offered him Legion of Honor, which soldier refused, and only asked for ammo so he can return to his home in Serbia. That cannon is still kept in War Museum (again literal translation, I don't know if this is name which French and English use) in Marseille. I tried to google to see if this is true, but couldn't find non-Serbian sources. I also visited Marseille Museum website, but couldn't do a lot because site is in French. Can anyone check if this really happened? Info that there is really Serbian cannon from WW1 in Marseille's Museum would prove it.
After doing a little digging, it seems to be at least believed by the Serbs... Nothing in English. Interestingly, there are at least two different types of gun pictured associated with Jevto. Unfortunately, no photos seem to come up of the specific gun in Marseilles.
The translation of 'mountain cannon' in English is 'mountain gun', they are small artillery pieces designed to be man-portable. One of the two mountain gun types I see associated seems to be a Skoda Model 15, which comes in at 600kg, and is designed to be disassembled for carriage in four loads: Man-portable (or mule-portable) is a bit of a loose term, these things are still pretty danged heavy. Even just the barrel/breech assembly on its own is going to be a very heavy load approaching a quarter ton. (It's worth noting that the Skoda was on the 'other side', but the Serbs did use some captured pieces. Most Serb artillery was French. In fact, this choice to take French instead of Skoda artillery in 1906 started the Serb/Austro-Hungarian Pig War) .
The other gun which pops up in association seems to be the appropriately named "De Bange" 80mm M1885, which was French. This seems even less likely, as the barrel and block together are about 430kg Add the carriage, and the whole shebang is about a ton. However, there is a shorter 'mountain gun' variant of the M1885, the photo of which does not show up, the entire gun and carriage together being but a mere 400kg all told. The barrel alone would be about 130kg, but wouldn't do anyone much good without the mount. Being 400kg including wheels, I find it unlikely anyone could have carried the gun on his shoulder, but he could have carried the trail and dragged the gun. Probably getting a little assistance from colleagues going over some obstacles. If this was the gun, though, it would have been a pointless effort as the 1885 was a well obsolete, slow-firing weapon and would have likely been jettisoned when the Serbian forces were re-equipped with more modern French equipment anyway.