Question

esotu em duvida destas frases:

whose horse is it? - de quem é aquele cavalo

porque ao inves de ''is it'' não é o that?
pois that significa aquele:
whose is that horse?

Answer 1

Your sentence is wrong. Forget everything and take a look at the example you yourself has just given, which is the correct form:

Whose is that horse?

Answer 2

Depende de qual maneira vc vai coniugar essa frase se esta no presente ou no futuro