"I see a beautiful horse."
Translation:Widzę pięknego konia.
Yes accusative, but koń is male, also animate, so it gets declined like the male genitive, also its adjective.
If you look the polish word up in wiktionary, it gives you the gender of a noun (as well as other useful info like whether a noun is animate, its declensions, and so forth)
Would something like "ładniego" instead of "pieknego" be correct as well?