If it was washed with soap before, just clean it very thoroughly with extremely hot water and NO SOAP and allow to air dry. Or even run it through the dishwasher on the sanitize cycle with NO SOAP.
You can't guarantee anything about what the manufacturer did with it, and I guarantee the manufacturing process is much nastier for the fish than using a little soap at home - so don't take any risks and just clean the thing with super hot water first! You might even do a vinegar bath, but I'd only bother with that if you thought you couldn't get the water hot enough.
Now, if there's a chip in the mug, don't use it - you never know what got soaked up inside the ceramic, whether from you or from the manufacturing process. Ceramic is porous, but once it's glazed with no chips it's fine if clean and sanitized.