Ok who are you and how did you get in my mine?

So I'm doing what I spend a lot of my time doing these days......playing Minecraft. Oh minecraft how oh how you take up my time. Any way I was walking around in my mine-house thing, its a house with a mine under it, so I'm in the mine on my way up after mining some ore. Iron I think and I hear a cow, yes a cow in my minehouse.....like that name its now my minehouse......its like a moon base, so in minehouse 1. Yes there is more than 1 a lot more that one, sooooo the cow yes this cow was just sitting in the opening of my mine part of the minehouse 1. Now this confused me because there well should be no way in to my house unless you can open doors FYI cows can't. Anyway me and this cow stare into each others eyes and i'm like "how are you in here" and the cows like "moooooo" cus well its a cow, not expecting much from it at all to be fair "mooooooo" was about all they do. So me and this cow look at each other a bit more before I smack the crap out of it, and it dies. So now I'm left in my house with the remains of this cow I just killed and a question who were you and how the hell did you get in my mine'. At this point I think well there must be a hole somewhere so off to look for it I go

No hole....

So re-cap there is a dead cow in my minehouse and no way for it to have gotten in...so I put this down to a random glitch so back down the mine for me and I go. Down and left and down and left and I get many shiny things for my troubles, and on the walk to my chest who is there but two chickens and a sheep......what first a cow and now some sheep and chickens what is this an arc of some kind...no I think not. They are killed off at double speed so back I go to look for the way there getting in, its not the portal to the nether I have, its like hell but with pigpeople, and there's still no hole.

No hole....

So were are they coming from....

I think this is one of the many things that I will never know about this game. The other is where are all of the squids hiding.  In fact I can hear a sheep in my house right now WHO ARE YOU AND HOW DID YOU GET IN MY MINE HOUSE that's it get my killing stick.......