yehaa.
I got it. The catching and losing it problem is widespread. I read many many posts from different sites, forums and such, but nobody had a solution, it was mainly lots of players saying it has happened to them too. I tried using a rowing boat that i found in ST.Denis and rowed it to the lighthouse in Van Horn. Spent a few full in game days fishing from the boat, but still no luck. I managed to get my horse on the little rock with me but still nothing. I did read somewhere the horse on the rock idea did not work for some reason or other. The way to do this if lost is go elsewhere and do other things for about 3 ingame days. then return to Van Horn Lighthouse and the little rock. save the game then DO A FULL EXIT GAME. When you load the game again return to the little rock and the Legendary Muskie is back.