I know kobolds are the standard minions for young evil dragons, but what about older ones? I like the idea of an ancient or older white dragon with a tribe of zombified frost giants guarding its lair (since frost giants and white dragons hate eachother I doubt they would work together willingly.)
It's situations like these that I kinda miss the preponderance of random tables being used in my games. I've largely gone away from them, preferring a more curated approach to encounter design, but sometimes random generation tools can be a great source of inspiration.
So, here's six ideas for cool older dragons guards I just thought up cause I was bored:
1) A group of Paladins who have deified the dragon and their 'ideals', and have sworn to guard it with their very lives.
2) Golem/Mechano-dragons, the product of engineering and creativity that the elder dragon has been fascinated with its entire life and studied profusely.
3) A local thieves guild who doesn't KNOW they are guarding the lair of a dragon. The guildmaster (the dragon) is only ever seen by it's right hand man (who has been ensorcelled/charmed by said dragon), and a portion of all the thieves treasure goes to increasing the size of the dragons hoard. All the benefit, none of the work!
4) A circle of Warlocks who all draw their power from the Dragon they have made a pact with...in return for it giving them a portion of its abilities, they keep it's lair safe.
5) The Dragon has mastered the art of Undeath, and has enslaved the souls of former heroes that challenged it to having to guard him (kinda like your zombified frost giants idea I guess)
6) There are no guards...the Dragon simply does not trust anyone enough to charge with protection of its lair. So instead, it has created a fiendishly complex labyrinth of mechanical and arcane traps the party must navigate...