so what happens if {reroute} is cast targeting {fleecemane lion}'s monstrous ability on the stack. would another creature get a one one counter?
{Reroute} won't affect it, as the ability itself does not target and thus cannot be retargeted.
RESOLVED
ok. is there anything that basically reroutes activated abilities regardless of targets?
No. You can never change the target of abilities that don't target. The best you can do is {stifle}.