ToME WikiKhymChanur: Two ideas for new types of Z hounds:
Chameleon hounds: Like clear hounds, except that they breath whatever is normally used by hounds of the color they currently are. So if it's standing on something violet it will breath nexus, something tan it will breath gravity, and so on.
Unformed hound: Starts off with no breath attack. However, the first time they take damage, they'll polymorph into whatever sort of hound does that damage, unless the damage is crush, slash or pierce.
PatashuPatashu: Why, what's wrong with crush hounds and slash hounds? 
Both ideas are cool but sound like they'd be a female dog to implement. That said, is a good ol' multi-hued hound implemented yet?
Oh, also, wouldn't the potential of a Chameleon hound vary depending on whether you used the 'map used special colours for torch-lit grids' or not?
RavenRed: AFAIK, no Z-hounds in T3, which will have a somewhat re-Tolkienised monster list. Don't let that stop you, though.
DarkGod: *nods*
KitaKita: How about Junkhounds, which spit out random skeletons, shards of pottery, and other ~ type items at you?
IngeborgNorden: Z-hounds may not exist in ToME proper when it's redone for T3, but that doesn't rule out other modules using them. (For what it's worth, my own mods are going to treat elemental hounds as another 'C'anine type, rather than giving them their own letter.)
As for hound breath weapons: I agree that some damage types don't make sense for those. But slash-damage breath already existed in T2 as 'shards'; piercing breath could be justified as 'thorns' or other small, pointed objects. I'd have a hard time finding a good description for crush damage, though... 
P.S. to KitaKita: I'd call that item-breather a Junkyard Dog (and probably flag it as a joke monster, but that's just my opinion).
ElCuGo: The Catapult hound breathes boulders. You are crushed!
Sirrocco: While we're at it, Vacuum hounds, for that good old unbreath damage. Also, I'm rather fond of the idea of "Avalanche hound" rather than catapult. Just sounds niftier. Oh - and lava hounds: they breathe with fire damage as normal, but the chance for their breath turnign the square you're on into lava (or deeper lava) is quite high. Likewise with submersion hounds, who breathe for wather damage and a high probability of making your/nearby squares water. For that matter, if they're hounds made of water/lava/whatever, make them invisible while in their home terrain type (perhaps even darkness hounds invisible in darkness/torchlight or Light hounds in permalight (though in darkness their own glow would give them away). Or... *wham*. (Sorry about that, folks. I just got bit by the Random Idea Fairy for a while there. Fortunately, I had a bat.)