I don’t know about possible legal restrictions, but I would advise against using a brand name that is already taken for your typeface, simply for reasons of practicality. When, as a potential customer, you search the internet for ‘Maiola’, ‘Gentium’ or ‘Grotext’, the typefaces come up as the first or one of the first results (at least for me on google.com). For ‘More’, ‘Scala’ or ‘Utopia’, that’s not the case. I’m not saying that the latter are bad typeface names (because you can always add ‘font’ to your query and get to what you want). I just think that it’s an added benefit if your typeface can have a name that’s both memorable, easy to pronounce in many languages and (relatively) unique. Before settling for something suboptimal, I would first try to explore if you can find a name that satisfies these requirements.

Picking a name is hard, sometimes very hard. Pick an interesting and perhaps unusual word, then try variations on it as a possibility. Look at the typeface and think about it; does it bring an image to mind that would be useful in naming it, a word or words that kind of describe the feel of it? Sometimes it can take days or even weeks to come up with a name you are happy with.

Just try to make sure it doesn't mean something ridiculous or offensive in another language. It also doesn't help much if it sounds like a medication or medical condition. Avoid words that can be associated with historical figures or places that have a very bad reputation; Hitler Bold, for instance.

Your example of Prius is something to think about. Prius is a Latin word meaning "before". Since you are not building cars and offering them for sale to the public I believe that since it is a normal word you could use it as it is for fonts.

You could call it a name like "Impala", for instance, and General Motors wouldn't likely complain because an impala is also an animal. Whatever you call it, if it is or sounds like a known trademark then tack on a descriptor such as Regular to it to make it more unique.

Unless you are a large company with a lot of money the most trouble you would get into is receiving a letter from Toyota's law firm demanding that you stop using the name "Prius Regular" for your fonts. They will not appear in person at your door to put you in handcuffs.

All that being said, I am not a lawyer, merely someone like you who has to come up with good names to use. Also like you, I don't go out of my way to invite trouble. Just use common sense and you'll be fine.

What other have said. I would add that it also depends on the renown/obscurity of the brand in question (I would advise against calling a typeface “Nescafé”, for instance). Lastly, make sure the name looks good when typeset using your font! ;)

A common name like apple can co-exist as a Beatles record company and a Steve Jobs computer company. But for a name with strong established associations you might have problems. For example I would avoid a font name like Disney. The safest course is a neologism like the ones that flood corporate identities, such as Exxon -- create your own, don't use this one. Or do a compound that differentiates your product from an established coffee brand, like VerdeBlueMountain. Try to stand out and be memorable, like VerdeJavaJolt or Verdeccino -- a wordplay on cappuccino.
Don

Guys, you posted some great answers, made good points and gave me a solid ground to work from. Thank you!