I can explain.
The Host: The Host was an old monster that was COMPLETELY reliant on AI. It wasn't very fun to play as because it's AI ability dependence, so she was scrapped. She was already built however, and because they connected with her so much, she's an official part of the Evolve universe. Maggie talks about her and Behemoth showing up on Factor. She was the "monster that makes other monsters". She just won't ever show up on Shear. And this is her.
Cupcakes: There are hidden cupcakes in ALMOST every map, and one in the Dropship. This is because they didn't have rag doll technology back in Evolve's early days of being built, so when you killed a hunter they became a giant cupcake. The next time you play Support, look to your right in the Dropship. There's one in a cubby that you can see right before you drop.
CMET End Credits: I'd get @skills4u2envy to answer this one. She understands it a lot better than anyone else.