How the Tales of Arcadia franchise and HTTYD franchise are related

Dreamworks has created MANY shows. But two franchises that are both fantasy, relate a lot to each other. Luckily for me they're both my favourite. I'm going to hint some references that maybe you'll see too!

The shows (in case you don't know) are:
Trollhunter and 3 below (Tales of Arcadia)

and

How to Train Your Dragon (HTTYD)