Indeed, some bosses have shown their ugly mugs time and time again. To see what forms they've taken in each of their appearances, click the link of the name you wish to view. Remember: This section is only for bosses who have appeared in three or more games in a similar incarnation. You'll notice that on the regular boss list that two separate enemy formations may appear--like Frankenstein and his Symphony version, the Creature--and that's due to my judging that they're different enough to warrant being mentioned twice, three times, etc. But here, regardless, they're all the same.
Some bosses have managed to retain extremely similar forms for only three or less games. In that vein, they miss being listed under any category. To see this select group of demons, click the link and behold.