Did they always change names when accessing the throne? Why the names? Why Peter? There's no Bulgarian tradition of Peter as a royal name. I find it weird. Kalojan has an understandable name from a Byzantine Imperial perspective. But there's no Emperor Peter that I'm aware of (it would be cool, though). Any insight on that?
As for strange names that make one's rule awesome, since I am somehow big on onomastics (onomastics and heraldry geekery is one of the geekest forms of weird), I've seen over the years that awesome people usually will try and make themselves stand out with awesome names, especially back when names were all people would hear of a ruler, if at all.
Examples include:
Pope Francis, who got himself a very unusual name in order to mark a very unusual papacy. A name, Francis, that didn't even exist before St Francis himself made it popular by adopting the name everybody called him (Frenchie, basically).
Kaloyan himself, his name being a nickname and the name he's offically known for is also awesome.
Napoleon. I mean, Napoleon! Strange Lombard name out of Early Medieval mysts turned into the epytome of military prowess, empire-building from scratch, self-glorification beyond measure and patron of both liberalism and tyranny. Napoleon! He even got the letter N to mean Napoleon for at least a century!
Emperor Alexander, the "last Pagan Byzantine Emperor", also probably not Pagan, but he was so reviled that he must have done something truly... remarkable.
The two almost King Arthurs in English history, first the nephew of King Richard the Lionheart, who was captured and allegedly murdered by King John Lackland, and the second, Henry VIII oldest brother. I'm still waiting for William to change his son's name to Arthur...
King Martin the Human. While his name was odd for Medieval royal Aragonese standards, his nickname is even weirder. It originally referred to his humanistic-ness and his goody nature. He had a son who was also named Martin, the Young, but died before Martin the Elder, so there was never a Martin II of Aragon.
King Francis of France. Granted, he was never considered remotely elegible for the French crown as a child, being the son of a nephew of a cousin or some such "many times removed" parentage with the then-king, that's why his name was not Louis (let's all remember that Louis XIV, the Sun King, named his son Louis and this Louis named three of his sons Louis in a row, after they all died as babies one after the other. There must always be a Louis in the French throne! That's why Francis being just Francis is kind of awesome. Also, Francis means Frenchie, so double points.
Jagiello. Yes, he changed his name to Wladislaw, much more conventional, but his name became also the stuff of legends. Jagiello, Jogaila, Iagellon, whatever the version.