Animal Companion

Provides a bonus to happiness at the cost of population upkeep. There is an associated slider where the player can control the prevalence of the animal and the bonus/cost associated. The slider can only go up so far each turn based on the current prevalence and only down so far based on the death rate. Limitations are based on the birth rate of the animal and the fact that pet owners are dying off respectively. The player can slide the bar as far as they want, but each turn it will only change as much as the limitations allow. Being able to slide it where you want just means you don't have to manually change it each turn.

In addition to happiness, animals can provide other benefits based on the animal type chosen. The animal chosen will than have a random chance every so often of mutating, possibly becoming better at their task or more or less resource efficient, effecting the upkeep cost.

The player can also devote more into the above with further advances, like the social tech gated ability to have multiple types of animal companions and some tools to manage where they are used. Or a genetics based way of having more control over the mutations that occur.