Maybe dolphins. There's a video I watched in a linguistics class that was investigating dolphin communication using dolphins that were trained to perform in dolphin shows with various hand signals from their trainers. They had a hand signal for "together" that typically was used to have the dolphins coordinate jumping out of the water or something similar at the same time. So the trainers would use hand signals to say "do a flip together" and the two dolphins would time it so they were flipping at the same time. Pretty cool but not anything crazy. They had another hand signal that basically told the dolphin to "make up a trick", when they used this hand signal typically the dolphin would do some kind of flip or jump or whatever they felt like doing. In this video they had 2 dolphins, and they combined hand signals to tell them "make up a trick together". You can see in the video the two dolphins swim down to the bottom of the pool, squeak at each other a bit, and then come up and do the exact same thing at the exact same time.

I think communication becomes language when you can use it to express new ideas, and this is the only example I've seen of that obviously displayed in animals. I'll see if I can find the video, it is really cool.