I have a ready for harvest black mushroom but so far its not been attracting much in the way of higher level mythical creatures. I notice a lot of other gardens having a fully grown black shroom are attracting the black unicorn contrary to what the wildlife guide says. Any tips?

RFH black mushroom can only attract the black unicorn. RFH mushroom only attract "their" myth. FG mushrooms, except Agaric, attract all the myths below.

As well as what Shannon said, it's always best to use high plates. When the guide says a critter can use any table, unless you definitely need a low table for something else you're attracting that needs to be lower one, high are always the best to eliminate 2 of the 3 mice. Also, if you really want something to come, it's best to just concentrate on that particular critter, especially rare ones, and give more chances of something turning up with more plates...1 plate of that particular food isn't enough to really give it a chance to turn up :)