What semantic notions underlie 駁 (to rebut) with 馬 (horses) and/or 爻 (I Ching's hexagrams)?
Two notes:


The original meaning of 「爻」 is criss-cross patterns, rather than I-Ching hexagrams;
「爻」 provides a phonetic hint to 「駁」.




「駁」 (Baxter-Sagart OC: /*pˤrawk/, patterend/variegated horse fur) is composed of semantic 「馬」 (horse) and phonetic 「爻」 (/*N-kˤraw/). 「爻」 (criss-cross patterns) simultaneously provides a meaning hint.

The semantic extension is as follows:


Variegated horse fur
Impure/mixed colours and patterns (looking like a jumbled mess)
To distinguish, rectify (e.g. in the process of sorting out a jumbled mess, correcting what is chaotic/wrong)
To rebut (e.g. to refute falsehoods, denounce wrongdoing)




References:


漢語多功能字庫
《漢語大詞典》