I was travelling through Hollowshade Moor and see him In the water myself. I killed him several times without getting his rare drop (Nice resist collar).
I seen him in the beastiary and his collar on the Hollowshade loot table, but the only times i've seen him up is during the Hollowshade War when the Grimling Camp is sonic wolf occupied. At the time the sonic wolves were also in their own cave as well. I think grimlings were in the owlbear caves.
I've ran the event a few times and scarflank isn't the main sonic wolf boss.
Hope this helps