that at a time when everybody was a connoisseur in horseflesh, the appearance of the aforesaid pony at Meung[.....]produced an unfavorable feeling, which extended to his rider. And this feeling had been more painfully perceived by young d’Artagnan (the rider) [......]from his not being able to conceal from himself the ridiculous appearance that such a steed gave him, good horseman as he was. He had sighed deeply, therefore, when accepting the gift of the pony from M.d’Artagnan the elder.He was not ignorant that such a beast was worth at least twenty livres
The Three Musketeers by Alexandre Dumas
What did he mean by the bold part? I understand as [ the reason why he perceived this feeling so painfully is that he wasn't able to hide the " the ridiculous appearance" his horse's color give to him. Giving What a great a horseman he is, he should be able to].