I thought I had a good idea about using Poison Ivy as a deterrent to Squirrels and Raccoons using the 6 foot cedar fences around my yard as a highway. The idea was to collect some growing along the creek smash some up and then rub the vine on the top of the pickets. Hopefully the Poison Ivy oil will cause them to itch and they would not come back. My hopes were dashed when I saw the following photo below. It seems most animals are not bothered by Poison Ivy:

Photo courtesy of Rachel Mathes.