My sister plays a ranger in my campaign and having a pet to fight by her side is her favourite part of her PC. She has a wolf, which I level up once in a while to help him keep up with the encounters.

Today, after a session, she asked me for more interesting companions she could get (and switch during fights) that also do something more interesting than bite.

I'd like to do that for her, but I'm not knowledgeable when it comes to creatures, I also only have Monster Manual.

What would be some nice and interesting creatures that could fight by her side? I do not care if RAW allows them, I can also adjust their power etc. I just need something that would be amazing to a person that doesn't really know much about D&D other than wanting to have fun with her zoo at my table.