I find I can communicate fairly easily with most animals when I "open up" and allow the communication :)
However, there is one of my cats that I can "talk to" all the time. I am a Witch, and he is my familiar. We have had an incredibly strong bond from the day he showed up (literally) on my doorstep. I can talk to him, out loud or mentally, and he will do exactly what I say/ask; there are also times I can "hear'" him talk to me, as well (mentally, like I am hearing his thoughts.) Even when I am not at home, sometimes I will suddenly hear him in my head!
I believe that most of us, at some point in history, had a much stronger ability to communicate with animals, but over time, we "lost" it, because it was no longer "necessary" for us.....