Did you see the jellyfish?
I remember reading somewhere Baikal hosts the world's only species of fresh water coelenterates. A species of jellyfish was stranded there when the lake was cut off from the world ocean, and they slowly adapted to fresh water as the lake gradually lost its saltiness. I understand a species of seal lives there too.
While surfing, looking for info on this beast, I stumbled onto this. It sounds like total gargonzola, but unlike most of this stuff, this was written by someone who apparently knows some real science. I submit it with no guarantee of its authenticity.
https://forum.agoraroad.com/index.php?threads/legend-of-lake-baikal-immortal-underwater-beast.3088/