Many years ago we agreed to look after 2 guinea pigs for 6 months whilst a family went out to Australia... a friend of my husband's did the house sitting and his wife looked after the 3 horses the family had.
They didn't come back. We ended up with the 2 guinea pigs permanently. One of the girls went blind whist we had her, so relied entirely on that other. When the other one died, we had to obtain another guinea pig rather quickly, as in next day. I found someone who had an adult one and we took the blind one with us to see if they would get along... they did.
All seemed well until we noticed that the new one could no longer get in and out of the barrier into the cage (draft barrier)... we had to move it further out, and further out and... yep. She was pregnant ... we ended up with a few more guinea pigs than planned!