So the spell find familiar, is the only options the creatures that it mentions or can it be anything? Asking because one of my PC used it and went to extras tab clicked familiar and a lot of other creatures popped up as a selection for a familiar, for example Adult Blue Dragon. So my question is is the spell only able to summon the choices it mentions like owl, cat, etc. or can it be anything?
The spell is limited to very low CR creatures. The spell calls out a specific list, but most DMs allow comparable creatures. Your DM may also, however, give opportunities for a character, or the party, to befriend basically anything out of the Monster Manual. In the case of the Adult Blue Dragon, the party could acquire it as an egg, and raise it from hatchling to maturity.
Or the party could enter into some sort of pact with it as an adult, to engage in some greater mission. Such an alliance is probably temporary, but is a situation that could occur.
I think an Adult Blue Dragon is a bit too powerful to include with Find Familiar, and even if you did raise one from a hatchling it wouldn't give you the benefits specific to Find Familiar (the ability to summon or dispel it at a whim, or to communicate telepathically and see through its senses).
I know that the Warlock Pact of the Chain allows for more powerful familiars than the default options, so doing anything beyond a common, Low CR creature would require something beyond the limits of the spell. I think you could probably homebrew something inspired by the spell Find Greater Steed (4th Level spell), which essentially gives you access to more diverse and powerful mounts than Find Steed (2nd level spell) is able to. Alternatively there are magic items that can summon loyal companions that function as familiars, and at that point the power of the creature is entirely up to the DMs discretion.
I don't think that Extras tab limits the creature selection based on what type you choose, I think it just determines what parameters to use to modify the stat block. For example, Ranger companions and Wild Shape choices involve changes to the default stat blocks of the creatures.
The familiar choices listed by the spell make up most of the creatures from the Basic Rules of type Beast, size Tiny, and Challenge Rating 0. It leaves out badger, scorpion, maybe a few others. I would say choosing any of the other creatures fitting those three parameters would be reasonable, though badger seems a bit large(even though it's techincally a Tiny creature). There have been two additions from other sources, the Cranium Rat and Tressym. Both of them have some nifty abilities that make them potentially more powerful than the standard familiar choices, but probably not game-breakingly so. The Storm King's Thunder text for the Tressym explicitly offers it as a familiar choice with the DM's permission.
So the spell find familiar, is the only options the creatures that it mentions or can it be anything? Asking because one of my PC used it and went to extras tab clicked familiar and a lot of other creatures popped up as a selection for a familiar, for example Adult Blue Dragon. So my question is is the spell only able to summon the choices it mentions like owl, cat, etc. or can it be anything?
The spell is limited to very low CR creatures. The spell calls out a specific list, but most DMs allow comparable creatures. Your DM may also, however, give opportunities for a character, or the party, to befriend basically anything out of the Monster Manual. In the case of the Adult Blue Dragon, the party could acquire it as an egg, and raise it from hatchling to maturity.
Or the party could enter into some sort of pact with it as an adult, to engage in some greater mission. Such an alliance is probably temporary, but is a situation that could occur.
I think an Adult Blue Dragon is a bit too powerful to include with Find Familiar, and even if you did raise one from a hatchling it wouldn't give you the benefits specific to Find Familiar (the ability to summon or dispel it at a whim, or to communicate telepathically and see through its senses).
I know that the Warlock Pact of the Chain allows for more powerful familiars than the default options, so doing anything beyond a common, Low CR creature would require something beyond the limits of the spell. I think you could probably homebrew something inspired by the spell Find Greater Steed (4th Level spell), which essentially gives you access to more diverse and powerful mounts than Find Steed (2nd level spell) is able to. Alternatively there are magic items that can summon loyal companions that function as familiars, and at that point the power of the creature is entirely up to the DMs discretion.
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An adult dragon, or particularly an ancient dragon is almost powerful enough that it could be a warlock patron rather than a familiar.
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I don't think that Extras tab limits the creature selection based on what type you choose, I think it just determines what parameters to use to modify the stat block. For example, Ranger companions and Wild Shape choices involve changes to the default stat blocks of the creatures.
The familiar choices listed by the spell make up most of the creatures from the Basic Rules of type Beast, size Tiny, and Challenge Rating 0. It leaves out badger, scorpion, maybe a few others. I would say choosing any of the other creatures fitting those three parameters would be reasonable, though badger seems a bit large(even though it's techincally a Tiny creature). There have been two additions from other sources, the Cranium Rat and Tressym. Both of them have some nifty abilities that make them potentially more powerful than the standard familiar choices, but probably not game-breakingly so. The Storm King's Thunder text for the Tressym explicitly offers it as a familiar choice with the DM's permission.
Short answer: the spell tells you what you can summon with it.