Let's Play X-COM Enforcer Mission 13
This mission is our introduction to the giant quadrupedal... things. I really have no idea what they're meant to be. They're noticeably tougher than anything else to this point outside bosses, anyway, and their combination of size and durability means that if you're careless they can easily pin you in a corner and crash your Hot Streak. They're not particularly dangerous in an immediate sense (Primarily by virtue of being a melee attacker), but they're the first enemy where just shooting at it isn't necessarily enough by itself. Especially since any weapons that don't travel through them lead to them meatshielding for the enemies that can shoot you. They're also uncommon, even on levels they do show up in.
The level itself is nice for its symmetry making it easier to parse, with its asymmetrical elements helping you keep straight which part you're in and what you've already done and so on.
I'm not entirely sure how the Stealth upgrade works. I know it can cause enemies to lose track of you, but the details beyond that? I've really no idea. It's an okay upgrade to buy, but unless the mechanics have some kind of exploit or something there's probably better stuff to buy at any given moment.