Avalanche is capable of being pluralized but I think it actually rolls more easily off the tongue as singular "Avalanche."
Kraken is both the singular and plural form of the noun.
Lightning is a mass noun. "Lightnings" is technically a word but sounds weird, whether the name of a sports team or not.
Wild can go either way. "Wilds" sounds strange for the name of a sports team, but a phrase like "into the wilds" is natural.
Mammoth is the outlier to me. I know what they're going for, but I still don't like it not being pluralized.
I don't really know why I wrote this response.