The thing 2011: original creature

Watching both films and always start with the 2011 film, and just got past the part where it gets henrik. What exactly was it doing under the building because the wiki says it was assimilating the dog, which the sound it makes would point to that. But why would it leave the dog’s body in the kennel and try and assimilate henrik if it was already assimilating the dog under the building if that’s what it was doing?

EDIT. Since I’ve gotta be more clear about my underlying questions as well.

What it was it really doing under the building?

If what it was doing under the building was assimilating the dog then how is it possible for it to assimilate multiple individuals at once if it’s already mid assimilation?