I think the deal with feral cats is they can sometimes carry disease, especially eye infections and conjuntivitis so it might just be something like that.

There are programs though for inmates to keep dogs and cats in their cells for training and to give them homes while they are up for adoption. I have seen it done in female prisons but I cant find any info for male facilities at least on the internet.

It would be something that your pal or pals would have to petition the warden about. Chances are he wont let it happen if they can't even have spiders. )-:

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"Thats it then, they will go on double, secret, probation..."