I've been trying out the new snapshot on an existing world and I've had a number of problems with a number of mobs becoming invisible, usually after teleporting. I've also ran into problems with my pet cats.

To begin with, I have 5 cats in my house who were all sitting and upon teleporting, a couple of them followed me, so I had to bring them back, which was difficult because sometimes I could tell they were in my house but I could not see them. Quitting to the home screen and reloading the world seems to help this. I then had to attach them all to a post with a lead so they could not follow me. They also seem to get stuck standing and would not sit no matter how much I right clicked them. Now they are all safely attached with leads and sitting, however, they will disappear at random times when I teleport myself to my house and usually stay that way until I reload the world. 

Additionally, I have sheep, horses, cows, etc. as well as a named enderman in my basement who will become invisible. I know these mobs are still there because I can hear them, and if I reload the game they reappear. 

I have also had similar issues with item frames and paintings disappearing. Again, these all usually happen when I've been away from my house and teleport back and they always show back up when I reload the game. It's not too big of a problem but it is annoying when I need to shave a sheep or something and have to reload the whole game because my sheep are invisible.