There's plenty of rares that are 20-29 as well as from 1-29 that you can choose from. And since your pet comes within 5 levels of your level when you tame it it makes it a little easier to make a choice of a lower level pet if you tame it when you're level 20 and level it up. Some options that are 20+ though, are:
Blind Hunter - lvl 27 elite/rare Bat, has a ranged stun and damage effect
Big Samras - lvl 27 rare Bear, has a swipe attack
Any of the elite boars in RFK ranging from 24-26, has a high damage gore attack (they have armor on them giving them a more unique look)
Vultros - lvl 26 elite/rare Carrion Bird, has an aoe demoralizing shout effect
Humar the Pridelord (personal favorite) - lvl 23 elite/rare Cat, has prowl and a bleed effect
Ghost Sabers - lvl 19-20 Spawned cats that have a ghost like effect, no real difference than other cats though
Barbed Crustacean - lvl 23 elite Crabs in BFD, has an immobilization and damage effect attack
Large Loch Crocolisk - lvl 22 Rare Crocolisk, has a retaliation like damage attack
Snort the Heckler - lvl 17 (not 20+ but the only rare hyena pre-30s) Rare Hyena, has a slowing attack
Deviate Viper - lvl 19-20 Elite Viper, summoned in the final portion of WC, has a ranged poison spit ability
Naraxis - lvl 27 Rare Spider, has the web immobilization ability
Leech Widow - lvl 24 Rare Spider
Chatter - lvl 23 Rare Spider
Creepthess - lvl 24 Rare Spider
Besseleth - lvl 21 Spider (has an interesting coloration)
Strider Clutchmother - lvl 20 rare Tallstrider, has an ability that causes someone's next attack to miss (aquamarine colored, there's also a lvl 9 tallstrider that's pink called Mazzranahce)
Kresh - lvl 20 elite Turtle, has a self-damage reduction ability
Ghamoo-ra - lvl 23 elite Turtle (formally the highest AC pet pre-pet nerfs)
Azzere the Skyblade (another personal favorite coloration-wise) - lvl 25 rare Wind Serpent, has a ranged nature damage attack (also worth mentioning are the white Wind Serpents in Thousand Needles if you're looking for unique colors)
Lupos - lvl 23 Rare Wolf, has a ranged and melee attack buff (prior to pet normalization and rare spawn nerfs he dealt shadow damage along with melee damage when attacking, making him ridiculously OP)
Misthowler - lvl 22 Wolf (I used to love this wolf on my hunter as his pursuit speed was one of the second highest for wolves, he looks badass too)
If you're looking for a highly comprehensive website with pet info I'd suggest checking here for more in depth pet information:
Petopia: A complete guide to hunter pets in the World of Warcraft.