"You have such a cute dog!"
Vi havas beletan hundon!
If you are suprised, say because you only have cats, then you could say:
What dog?
Ĉu hundon?
(it is a bit what you looked for when you mentioned kio hundo?)
Maybe you have many dogs and you'd like to know which one is the cute one, then:
Which dog?
Kiun hundon?
You will be understood if you ask:
Kiu hundo?
But it doesn't feel right, not to mirror the -n
ending in the question if you don't make it a full sentence.
For example, this could be a dialog:
- Vi havas ion moviĝantan sur la kapo!!
- Kion?
or
- Atentu! Vi havas beston ĉedorse!
- Neee, kiun beston?
If the question does not use -n
then you would answer without it (if you are not rephrasing it):
- Estas io inter viaj dentoj...
- Kio? Ĉu laktuko?
See this part of a play by Zamenhof (La rabistoj):
Maljuna Moor. –
Kompaton! ... en la nuna minuto
oni juĝas mian infanon!
Rabisto Moor, ektimigite. –
Kiun infanon?