"see" is one of those verbs that are usually not used in the continuous aspect in English in certain meanings.
We say, "I see the mountain!" (not: I am seeing); "I have a fever" (not: I am having); "I feel good" (usually not: I am feeling); "I believe you" (not: I am believing); "Do you understand this sentence?" (not: are you understanding).
Not quite certain how this would apply to a dog, but there is "to be seeing" in English, but always in an idiomatic sense, as far as I know. Like "to be seeing things" meaning "to see things that aren't actually there / to see ghosts", or "to be seeing somebody" meaning "to have a romantic/sexual relationship with somebody".
I have the impression, "hangi" implies the accusative suffix for the corresponding noun like a definite article would (if there was one) - given it appears in an accusative clause - since in that construction you are also talking about a specific instance, even if you obviously do not know, which one. Is that correct?
So "hangi" implies a specific object in the same way that "bu" might, even though "hangi" implies a question. That's why it's "hangi kopeğ i"? So, for example, "Which girl do you see?" would be "Hangi kızı görüyorsun?" And "Which vegetables are you eating?" would be "Hangi sebzeleri yiyorsun?" Yes?