Anecdotally, We got a couple of Bengal cats a few years ago. We got into the habit of saying 'Hello <cat-name>' every time a cat walked into the room.

Now the cats say 'eh-owww' whenever they see us, and also say the same phrase in different ways depending on what they want/need. It took us a while to work out the meanings of the intonations, but now we are pretty good at working out if it's a greeting, request, or just acknowledgement.

I'm not sure who trained who, but it seems to work. :D

Let's say, all those where some individuals manage to adopt human speech, like various birds, some whales, chimps, etc, or manage to interact with human language symbols by means of sign language or similar. This is quite impressive. To my knowledge, we, while far better equipped, haven't intruded a single communication system of an other species in a way that we could meaningful interact or even use their intrinsic concepts. On a less complex scale, but still impressive are those, who manage to interpret some human practices, like pointing. (Mind that this requires some kind of mirroring to construct an assumed line of sight and some assumptions about what the other might have in mind and what might be an object of interest and engaging in some sort of dialog over this.)

dogs