Mr. X- For when Umbrella wants to at least attempt to be a bit covert in their "disposal" operations (a photo of Mr. X could be confused for simply being a tall Arnold Schwarzenegger type dude by the media).

Nemesis- When Umbrella remembers a glorified SWAT team blew up their shit and need something that will keep moving even when it's just a single appendage so they say "fuck it" to any sense of subtlety.

And they gave their MEGA FUCKER an Bazooka just in case the SWAT boys call a helicopter, and have the bad idea of stay alive.