Explanation
A business rule can be used to configure the visibility of a field based on the value selected in another field. In this case, a business rule could be created to make the text field for additional pet details visible when the
"Other" option is selected in the dropdown field for the type of pet. This can be done by going to the Power Apps portal, navigating to the Common Data Service, and creating a new business rule for the entity that contains the appointment form. Within the rule, you can set the visibility of the text field to be dependent on the value selected in the dropdown field. Reference:
https://docs.microsoft.com/en-us/power-platform/model-driven-apps/model-driven-business-rules It's important to note that the above feature is only available in the PowerApps Model Driven App and not in Canvas App.