Translation:Kateřina is not thinking about the last horse.
-iho is in accusative the correct ending for this adjective because the noun (horse) kůň belongs to gender category living male.
"Kateřina doesn't think about the last horse."? "Kateřina doesn't think of the last horse."?
is this how sentences like this work? if it were crudely translated would it be "katerina about last horse (subject) is not thinking"
It is a different ending... used for soft declination adjectives (like poslední). -ého is a masculine ending for hard declination adjectives.