Drakkar (found MS-DOS port of Spanish arcade game; 1990)
The Spectrum and Amstrad CPC versions of the game had been made available sometime before the search began. In 2016, the MSX version was also found and uploaded on computeremuzone.com.
The MS-DOS port of the game is documented on computeremuzone.com. Additionally, the MS-DOS port of the game is also mentioned in the game's manual and six different advertisements. All these pieces of evidence are uploaded and preserved below.
On December 31st, 2017, old-games.ru user "Uka" uploaded a manual, screenshots, and floppy disc for the MS-DOS port of Drakkar to the internet.