https://s.castle.xyz/7CMuhBS4TOsS

This is because in the lore there's a loop where basically if something goes wrong a genocide route happens and it creates zombies so sans tries to break the loop by doing an expirement he locks himself in a prison until he went insane and then did a genocide route

So in ruin encounter he has a knife and in snowdin encounter he has a eye that gives him ice powers and in waterfall encounter he has a spear and pan on his head and in Holland he's just a cowboy and in the last encounter he went completely insane and got human eyes