I've been trying to find a furry version for Markus for a long time. The fact is that in some versions of his history and in AUs, different furry forms correspond to its essence and life path. If you read my thoughts here: https://www.furaffinity.net/view/38407065/, then you know that the fursona form is not constant for me :)
In the end, I drew a horse fursona so far, but I hope to someday make a snake and a swan, for a complete set.
Who knows, maybe someday I'll stop at one of them. But not today :D