Does welche/welches/welcher, the only question word which consider the masculine/feminine/neuter of the word which follows...? I wonder if there's any particular reason behind it.
I think I haven't seen other question words (such as wie, wo, wer) to adjust itself with the masculine/feminine/neuter thing.
It is welchES because Pferd is neuter and in accusative case. Right?
Can Welches translate to both which and what? Which and what have different meanings. "Which" is from a selection of things present or known. "What" is the unknown. So you could use what in place of which (bad grammar, as in doing so you are needlessly removing information from the sentence), but never the reverse.