Could anyone please explain why we have "snelle konijn" and not "snel konijn" as "konijn" is the "het" word?
Because the word “my” is before rabbit. In the tips section of adjective basics you can see the rules for when an adjective gets an e ending. Regardless if the word is a het or de word, if a demonstrative or possessive pronoun (like “my” or “this”) comes before the word it will get the e ending
Mijn snelle konijn
Mijn snelle hond
Both would require snelle instead of snel
Hope that makes sense!