You can buy the Rashberry at Miss Petula's shop, but you have to be a lvl 12 gardener first.

Feed the Rashberry a cheesecake and it will turn into a Hoghurt.

The site in the links has some pretty good info.
Source(s):
http://pinataisland.info/viva/Rashberry
http://pinataisland.info/viva/Hoghurt

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For a Rashberry:

To get it to appear, you need:
A gooseberry bush in your garden
A chili plant in your garden

For it to visit, you need:
Three gooseberry bushes
Two chilis

For residency, you need:
For him to eat two rotten gooseberries
For him to eat two rotten chilis

When the Rashberry becomes a resident, tinker some Cheese to create Cheesecake. Feed that to the Rashberry and it'll turn into a Hoghurt.
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