Place the cake on a clean work surface. Cut a 3/4-inch triangle from each side of cake, 2-1/2 inches from top of cake; reserve. These are the owl's ears. Cut a 1-1/2-inch slice off bottom of cake; reserve. This will be used to support the cake. Spoon 1 teaspoon vanilla frosting into a resealable bag and snip off one corner; set aside. Spread remaining vanilla frosting over owl's chest area.
Stand cake up on serving platter, cut end down. Attach the reserved 1-1/2-inch cake piece to the back with some caramel frosting to help support cake. Spoon remaining caramel frosting into a resealable plastic bag; snip off one corner. Spread some of the chocolate frosting over the head area and attach the ears. Pipe fronts of ears with caramel frosting, and smooth with a small knife.
Spoon the remaining chocolate frosting into a pastry bag fitted with a leaf tip. Starting at the base of the cake, pipe frosting to create "feathers," pulling the pastry tip away from the cake while squeezing out the frosting.
With the caramel frosting, pipe several dots onto the owl's chest.
Twist off tops of Oreo cookies to expose cream filling. Press cookies, with filling facing out, onto the owl's head. Pipe vanilla frosting onto filling, and attach Junior Mints for the pupils. Pipe a dot of vanilla frosting onto each mint. Attach Runt candies for the feet and beak. Break pretzel rods into varied lengths and place near feet for branches.