For Warlock characters that gain the Find Familiar spell as a ritual through their Book of Ancient Secrets Invocation, the Familiars section of the Character Sheet's Extras menu provides no restrictions of creature options, as dictated by the spell. 

This is by design to allow for variant rules.

It is up to you to select the correct creature. The use of the Familiar category is so you can edit creature type and so it adds the Familiar traits.

Hey, you could be in a campaign where you play as demigods and could totally have an adult red dragon as a familiar. Well, the Extra menu allows that, without needing awkward homebrewed workarounds.

Oh, I figured it would have been like the Artificer's Battle smith Defender, where there's a check box for only showing valid steel defender option, which you can untick to bypass them.