We know they're mammals. I think the point here is that the evolutions that happened to a few creatures ended up resulting in them becoming pretty bizarre.
Even though it's still a mammal, their uniqueness in some aspects make them extremely weird. Giraffes have the long neck, and the previously...
well that certainly explains why they're poisonous beaver-ducks that use their electroreception to hunt and one of the two modern mammals that lay eggs and squirt milk through the skin!
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