Is äter typically used for animals?
No, for eating in general.
Oh I ment to ask if there's not a separate word for it like in German (essen = people, fressen = animals)
Aha! No, äter is for everyone. :)
So whenever the word ends with "en", is there a definite article in front of the word?
The suffix -en acts like a definite article for words in Swedish, and there would be a definite article in an English translation.