I feel like taking the Kaiju here is a bit of a copout. Most of the mascots in the league are either normal animals, humans, or a select few vehicles and two fantastical creatures/cryptids. So instead I'm going to take one animal that is insanely scary by itself and give it a boost in an area it lacks. Take the sabertooth tiger from San Jose Sabercats, a now-extinct predator that was very rightfully feared in it's time. Nowadays, tigers and bears are still incredibly dangerous, but they pale in comparison to the force of a wild sabertooth tiger. Sabertooths are big, bulky creatures that will charge you at full speed. What's more scary than that? The answer? One that's silent. Give the sabertooth the element of surprise with some incorporeality (that ideally can be toggled on and off, because otherwise I've just made something that works great in a haunted house and not much else). The terrain and manmade structure that would have slowed or stopped our beast before become laughable when the sabertooth can phase through solid stone. Nobody tell Australia about this, I'm confident that if they know of a concept it just spawns there.