This guide can help: https://www.fairylandgame.com/fbfairy/forums.php?tid=116313
This can help too: http://www.freedomfairies.com/green-topiary-trail.html

Also, the green trial will eventually attract all wildlife green and lower (grey squirrl, skink, unicorn, meerkat, tortoise, winged uni, ostrich, hedgehog, black uni, giraffe, woodpecker, & white uni). The order of the wildlife will be random (you can use this thread to help figure out the order with some accuracy although not 100%: https://www.fairylandgame.com/fbfairy/forums.php?tid=131843), but the last task will always be the highest level unicorn, so for the green trial it will always be the white unicorn.