Welcome to BYC
Chickens are social creatures and you cannot keep one alone. Get birds of the same size, if you can. Quarantine them
for 30 days minimum, then put them in a crate/sectioned off erea inside the coop with the other bird. This will alow them
to see and talk to eat other, but no pecking. After about a week of this you can let them out and see how it goes under your
supervision. They are going to need to sort out the pecking order, and theres going to be pecking most likely, but unless skin
is being broken - don't intervene. let them sort it out. Some people also go outside and add the new birds to the flock when
it's dark outside, and everybody in the flock is asleep. I, myself have done this with great results. If we're talking chicks,
seperate a part of the brooder for the new ones where they and the chick you have now can see each other, after a few days,
take off the sectioned off erea. Even if they are week olds, quarantine and try and get chicks of the same age. In chicks, A
week or two difference in age is the most I'd go. I can't help to wonder, did you sort out the dog problem? Unless they have
been fenced, or 'taken care of', they'll be back. No doupt about it. Very sorry about your lost birds.
Ukiah