You should keep them in a pen, and protect them if they're in a dangerous environment.

I had some in the plains, behind double stone walls, and a fuling got in somehow and killed all but a single piglet. After growing them back, a deathsquito wiped them all out.

If you're in the meadows, just put a cheap wooden fence around them to keep them together and it should be fine. You do need to "harvest" them or else they will not produce anymore.

i had a 1 star and a 2 star and bred them till i had 2 star pigs. then i let the 1 star go because i didn't know you had to enable PvP pre-tin knife to kill them. the 1 star is gone somewhere. if it died i have no clue.

Boars should stay in a pen, or run arround free in a Protected base, but away from all kinds of fire (all except these Halloween lights frighten them = no breeding)

Wolfes should stay in a pen, or run arround free in and arround the base, but i highly suggest the mod that silence them

Lox should be in a pen made of walls of soil, away from critical base elements, their attack is a AOE that can destroy your base

If you want a meat and wolf factory the best you can do is a breeding tower

Thanks guys.
I am in starting area still. Only thing I seem to have is Boars and Greylings. I'll keep em penned and start making more pens so they keep breeding.

Just found one of my boars wandering around my farm, which is quite a ways away from my boar pen, and over a wood fence and double stone wall. Sometimes they spawn outside of containment. The point being that they can wander quite far.

I've had them even spawn into the wall with just their head showing! :-)

When it comes to pens I suggest as you get higher adding a roof to them. I do a stone wall with a core wood 'roof' with a real roof over that. I do this because 1 drake from an event can totally wipe out your ranch.

As for the animals being found outside of your pen... yeah they are escape artists. Like hamsters, own a hamster and you will know what I mean. :)