Chewnicorn is one of the harder piñatas to obtain, using this method it'll help speed up the long process but a lot of it is down to waiting for one to turn up and when it does hoping it eats some gems!Appear requirements;
To make a Chewnicorn appear you'll need to have obtained the "Horstachio Master Romancer Award"
, to get this you'll need to successfully breed your Horstachios 5 times so that you have 7 of them in your garden, alternatively if you have the chocolate coins to spare you can pay Gretchen Fetchem to catch a Horstachio for you, repeat this process until you have 7 of them in your garden and you'll still receive the "Horstachio Master Romancer Award"
, for more information on how to attract a Horstachio to you garden see here; http://pinataisland.info/viva/HorstachioVisit requirements;
Once you've attracted the Chewnicorn to come close to your garden the next step is to make it want to visit your garden, to do this you need to have a gem tree in your garden, now while the trees maturity doesn't matter for this stage you should grow it to full maturity as you'll need to for the next step.Resident requirements;
To make a Chewnicorn want to become a resident of your garden you'll need to have a gem tree which is grown to full maturity, this is easier then it sounds, while growing the gem tree keep it watered, hopefully by the time you're going for this achievement you'll have access to the watering can which you only need to use once to keep a plant well watered through it's entire growth, you'll also want to visit Ivor Bargain and purchase his special fertilizer, use this on the tree as it grows, doing this will make the tree grow to it's full maturity. The Chewnicown will also need to eat 15 gems from the gem tree, there's no fast way of making him eat these gems, he'll just eat them as he pleases.
Alternatively if you have a friend who already has a Chewnicorn they can send one to you over Xbox LIVE which will save you having to get one yourself, unfortunately not everyone can get hold of one so easy and if you can't use the above method, it might take some time but you will get one eventually.
Now, once you have a Chewnicorn the rest of the achievement is easy, you need to make use of the Chewnicorns special ability of healing fellow piñatas, to make a piñata sick by letting it eat sour candy, sour candies are dropped by Ruffians and the sour piñatas, you can also feed a weed to a piñata to make it sick.
Once a piñata is sick select the Chewnicorn by pressing "A" on it and direct it to the sick piñata, the Chewnicorn will then approach it and heal it which will unlock the achievement!
An example of a Chewnicorn healing a sick piñata can be seen in the video below.* Credit to AnInsaneGuy for the video