In general, a trademark claim may be filed by a party that holds a trademark on a brand name (such as "Ugg") either because another party is using the exact name they have a trademark on and/or another party is using a name or mark that can "confuse" or "mislead" the public into thinking that the item or product they are selling is manufactured by or associated with the party who owns the trademark.
You would need to have an attorney who works in intellectual property law or patent law review your designs and compare them to the 50+ trademark registrations owned by Ugg, which you can see by searching the US Patent and Trademark Office's online search (http://tess2.uspto.gov/bin/gate.exe?f=tess&state=bblnfu.1.1) to determine if the use is a direct violation, how easy it would be for consumers to be mislead or potentialy mislead, what the likelihood of a lawsuit would be, whether you can use the name, and whether you would need to make any changes to your design or name to allow you to use it and what those changes might be.
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