Are there any beer styles that are best enjoyed hot? I'm well aware that beer is commonly enjoyed warm in Europe. However, mulled beer aside, are there any beer styles that fare well when enjoyed hot?
First, to clear up the myth.   European beer is not served warm.  Some beers, such as real ale (or cask ale) are served at cellar temperature, which while certainly above the temperature of a beer fresh out a refrigerator, at 12-14 degrees celsius (53-57F) is still much colder than room temperature.  

To answer your question:  Mulled beer is heated beer.  As with wine, mulled beer is usually warmed over a low stove or in a crockpot with spices, often including cinnamon, nutmeg, citrus zest, and a sweetener like brown sugar, but it doesn't need to include any additional ingredients, particularly if it already has strong spice or fruit characteristics.  So, any beer style that you heat is, in fact, mulled beer.     

As to styles of beer that would be suitable for mulling, Belgian Strong Dark ales would be good candidates, as would nice fruity lambics.   Barleywine ales, milk stouts, chocolate stouts, and oatmeal stouts I would imagine might also be nice.  The characteristics I'd look for would be a heavy body, more malty than hoppy, a bit of sweetness, and if present, dark fruit (plumbs, figs, raisins) or berries.