Frisk... well... I was young and it was either going to be Jack or Frisk. I don't remember it that well, but I suppose it ended up being Frisk!
Ethan... when we adopted him, he had no name. I always like the name Ein, and my family did, too. I have a cousin named Ethan, and mum just wouldn't get used to calling the new dogs Ein, so it ended up being Ethan.
Skylar... my sister came up with it. Not sure how, though. Probably because of his blue eye.