Posts are 8’ oc or a little less depending on line length. No more than 8 because boards are 16’.
My initial thought was tape on stand offs on the posts, so it would be 2-3" proud from the boards. Tape on regular insulators is not proud from the boards, it’s “inside” the fence, and I’m just not sure how that goes since horses could be touching rails before they’re touching tape.
Wire or that black twine @Leather found could go on stand offs and that could definitely be the answer. Mucho easier to find stand off options in black for that type of product.
Or tape could go on regular insulators on the second board of the four board, so it’s still proud (ish) from the fence, but it’ll need to go below the top board on the 3 board sections (“inside” the fence, maybe it’s less of an issue with the wider board spacing, idk.)
I think that’s about the run down of options at this point. I really might pick up some pieces and try it out on the old fence to see what it looks like and gauge the deer thing. Kind of an investment but I really don’t want to be wrong on the new fence.