Something that I noticed when looking up information on all the legatus's we know, including Nael Van Darmus and Gaius Van Baelsar, is that while Nael and Gaius have nicknames ("White Raven" and "Black Wolf") Regula and Zenos don't seem to have nicknames as well in the same vein.
I know it's just me being nitpicky, but it does make me wonder why the developers didn't give the same sort of monikers to the latter two.
Nael and Gaius nicknames were earned through their conquests/leadership.
Regula was promoted into that position, and while he may or may not have earned it, didn't have the history and respect Gaius had, especially with his own men. Zenos is much younger than the others, and isn't well liked by his men so he wouldn't have earned a nickname like those two.
The lore book specifically states:
'Regula has had two great loves over his forty-four summers - music and ancient Garlean techniques of the sword. Few more ardently support the art and fewer still are more practiced.'
We can take from that, then, that Regula was indeed worthy of his position as Legatus. He further proved as much by displaying a strong sense of honour and through his noble sacrifice in the defence of an enemy. It also states, in regards to his weapon referred to as 'The Bastard':
'Regula prefers to simply call his arm what it is: a bastard sword with a firearm attached.'
If he is not fond of nicknames for weaponry then that may explain the lack of a nickname for himself.
Those nicknames for the person aren't bestowed by themselves.
Regula was perceived to have got his position from nepotism regardless of how worthy he was, hence no nickname from his own people. He also did not have the same level foreign conquests like Gaius had, so no nickname from his enemies.