Buttercup has some service dog training. She was intended as a gift from Natasha Stark to Bucky Barnes, but the message was poorly conveyed. Bucky assumes that Buttercup is Natasha's dog that he's now responsible for.
When Natasha is overworking on projects, Bucky will occasionally leave Buttercup with her as a reminder that living creatures require sleep, food, and water. He will then bring them both food and water, because he doesn't trust Natasha to remember to feed their dog.