"They are eating the horses."
Translation:De spiser hestene.
It says "They are eating the horses". Some people eat horses. It is common in France and Italy. US cavalrymen curried and ate their horses in the Philippines before surrendering to the Japanese. German soldiers ate horses when surrounded at Stalingrad. You can afford to be vegetarians and vegans because you have access to unlimited food of many kinds.
However, in this sentence, "They are eating the horses" does not mean that people are eating horses. "They" could be wolves, or as this is Duolingo, dragons or ducks.
The definite plural is typically formed adding "-ne" to the indefinite plural, which is usually formed adding "-e" or "-er" to the indefinite singular. So, if the indefinite plural ends in "-er", you'll have the definite ending in "-erne". In the case of horse: singular "en hest"; plural "heste", thus definite plural "hestene". Those are rules of thumb; then you have some cases where the indefinite plural doesn't change, or some English loanwords (eg. sandwich, chip) which do the plural AS IN ENGLISH (sigh), and those usually do the definite plural in "-ene".
That's what I got after some quick googling :-) Feel free to correct.