I noticed this too but for me it's a limit on options if you buy out one of the other animals the missing one will appear.
When I thirst noticed it there were five options, so I thought it the limit, but the other village had five types of animals, and there were only four options. Yes, it is possible to unlock the remaining one by buying one of others, but I think it still is a bug, which prevents the last option to show up.
There is a list of animals, and before the ability to buy a size-varied animals was implemented, the animals were presented in a strict, hidden order. If the first available to buy animal was removed by the missing animal quest, and then you brought it back, it was pushed to the end of the list, and was available for sale last. I believe that a similar list is still used, and probably the last animal is not shown for some reason.