Warning! They do indeed spawn at beaches and will murder you and everything you love. If you see one even about semi-far away from your base, and unless you have metal battlements (easy drop from beavers on mobile) get on a raft and move the flip away. They just move close and closer to your base, limiting the safe area in which you can gather stuff.
I got DESTROYED by a mate boosted level 125 while wearing Ascendant & Mastercraft Flak Armor (1500 Total Armor). Avoid these on foot at all costs. If your character doesn't have high movement speed, you probably won't survive. The Ark Freddy Krueger