Translation:Het is een langzaam schaap.
I thought we're supposed to use the adjective ending in "e" when the adjective is assigned before the noun?
It depends on the gender of the noun. Het words don't get the -e when an indefinite article is used.
It depends on the grammatical gender of the word.
Sadly, there are no rules we can follow, just some guidelines (like plurals always take de and diminutives always take het, among other general guidelines).
Make sure you read the Grammar Tips and notes, it's all explained there.
Otherwise, you can search on www.dutchgrammar.com