How common is it for toddlers to first adopt the behaviours and mannerisms of the house pet, before they become more human?

I’ve never seen that.

I haven’t seen that either.

Not typical

The toddler, or the pet?

Mannerisms, no.

But children do pretend to be animals. As they learn about other animals they may pretend to be them in play. But the first ‘role models’ of animals are the pets they see around them. And, of course, Disney movies.

I pretended to be a dog or cat quite often up until first grade.

I’ve seen kids crawl up next to a dog drinking its water and butts the dog’s head out of the way so the can drink they can lap up the water themselves. I’ve also seen them on hands and knees eating the dog’s food out of the dog’s bowl. I’ve seen owners trying to train their dog to dance while begging for food and their little girl mimics the dog by pulling up her hands like paws and twirling in as circle like the dog did. All of these were on youtube but the parents were entertained.
This one is the child mimicking the dogs Sitting . I couldn’t find the others I mentioned. Don’t know if they have been removed or if I didn’t enter the appropriate key words in my search.

Don’t think that it’s all that common. Just idiots hoping their video will go viral

No