Translation:Are you looking for big pigs, big sheep, or big horses?
The basic (nominative case, singular) form of the noun is kůň. It is a masculine animate noun.
Here we need the ACCUSATIVE plural form, which is, as we see at the top of the page, koně. (It can be confusing, because that is also the nominative plural form...)
The appropriate form of the adjective velký for a masculine animate noun in the accusative plural is velké. The form velcí is the NOMINATIVE plural form for a masculine animate noun.
Please see the Tips & Notes for more information. They are available on a desktop or mobile browser, but not currently in the apps.
Ovce, confusingly (at least at first) is both singular and plural. We know it is plural in this sentence, precisely because of velké. If it were singular, the adjective would be velkou. Link to declension table for ovce: https://en.wiktionary.org/wiki/ovce#Declension. If the table is not immediately visible, click Declension/Show. (This is a useful site to add to your resource list.)