Hermetic Cats (RoPM, p.70) have a long history in the Order of Hermes. The three lineages trace back "through generations of familiars...to the familiars of the Founders or their earliest apprentices". Two of the lineages specifically state that a Founder had a cat familiar.
Jerbiton's familiar was "a prince of the Egyptian cats" that gave raise to the Black Lineage and Mercere's familiar gave raise to the White Lineage. Then you have the Tortoiseshell Lineage which is associated with Celtic spellcasters from before the formation of the Order. Part of their history is most likely linked to Diedne, though there is a split into two branches of this Lineage that occurred following the Schism War.
AM takes place before cats were associated with Satan by the Pope and the mass cat killings, which are classified as an indirect cause of the Black Death. At the AM start date cats are still generally considered very useful animals for communities to keep.
So we have an animal that is directly associated with several of the founders, has several lineages that are found at a significant percentage of Covenants, and see themselves as part of the Order. That association with the Order and history of being familiars also means that Hermetic Cats would be much more willing to become a familiar. While some Covenants might have other animals (Ravens of Mistridge) which are commonly willing to become familiars, none of those are nearly as wide spread as Hermetic Cats.
For any Magus looking for an animal with Might:
The most likely first one they would find is a Hermetic Cat.
The most likely first willing to accept being a familiar is a Hermetic Cat.
That are friendly with humans, Hermetic Cats very much are.
That has INT instead of CUN, all Hermetic Cats do.
That have a broad range of personality and powers, which can be partially selected for by choosing a specific lineage. You can find one that fits many Techniques and Forms.
That have a reasonable Might, making them fairly easy to bind. Hermetic Cats have a Familiar Bonding Level of ~23. That actually puts them within reach of many Magi just out of apprenticeship.
That can help directly in the Lab, two of the lineages commonly have the ability to shift into tool using forms.
That blends into the surrounding, a Hermetic Cat would be ignored or get a generally positive reaction from almost everyone.
That can be surprisingly effective in combat and assassination. Invisibility, ability to grow larger, have claws like steel, and fur like armor are all fairly common powers among Hermetic Cats.
Yes they are not a "Unique" animal to take as a familiar, but most (npc) Magi would not be looking for something unique with a crazy high Might. Something that fits almost all of the check boxes, was commonly available, and has a long history of serving as familiars would be the most common selection.
While cats, especially those from the three mentioned lineages, would certainly be among the more common familiars in the Order, I am not sure they are the most common.
A strong contender for most common animal as familiar would be wolves. They are the favored familiars of House Tremere - especially the white wolves descended from their founder's familiar.
There is not exactly any shortage of magical wolves throughout Mythic Europe, and even without magical powers a wolf is fairly capable in combat.
Anything that is large(ish) and generally considered dangerous is unlikely to be the most common familiar.
Wolves are:
Large(ish) at Size 0, larger than the common Dog range of Size -2 and -1.
At least semi-Exotic, something you would not find willing co-habituating with humans.
Dangerous, with the Medieval perception of them as "wanton, bloodthirsty killers". As published they have Compulsion (killing) and a reputation Bloodthirsty 4. Having one as a familiar is open invitation for attempts to kill it. [Wolves in game are different from IRL, based on common understanding of them at the time.]
Linked to one of the most disliked Houses, which means Magi not of that House are less likely to take them as a familiar even if they had no other issues.
So no, wolves are not a strong contender for the most common animal as familiar. Only in House Tremere.
I'd say ravens and owls would be strong contenders for that title. Perhaps falcons too. They make great scouts, and certainly don't draw as much attention as wolves.
Yes ravens and owls would be strong contenders. As I mentioned, there are some Covenants that even have groups of ones of Virtue. Not as widely spread as Hermetic Cats but still strong contenders.
As the type of animal can be linked to the Technique and/or form and the strongest TeFo of the magus/maga is the most important thing I think we will see much more varied types than "cats live nearby I'll befriend one of them." That said, there might be a very slight uptick for cats, domesticated (or domesticatable) birds, dogs, wolves, and Poisonous animals are probably fairly common among Apromor Flambeau. IME this is much more of a factor and while characters don't "know the rules" it would be relatively common knowledge that finding an animal that aligns to your best arts is the way to go when it comes to familiars.
"A Technique or Form is appropriate if it corresponds in some way to the animal or its powers." There are many seemingly inappropriate animals by type which would make a perfectly valid Familiar for a TeFo combination because of their powers. If you look at the traits and common powers for the varies Hermetic Cat lineages, you will find that they they cover a wide range of Arts.
Also saying "cats live nearby I'll befriend one of them" does a disservice to Hermetic Cats. There are 'magical cats' that are not Hermetic Cats. By raw numbers in ME, they most likely out number Hermetic Cats (though not around Covenants). But those 'magical cats' are not Hermetic Cats, lacking their history which ties them to the Order and several of its founders. Mutts verse Pure Bred.
Yup. Even neglecting their powers, every one of those cat lineages can be bound with Animal, Corpus, Mentem, or Vim. Depending on which one, you can get Intellego, Muto, or Perdo. And then there are their powers, which could potentially add Auram, Herbam, Imagine, Terram, or Vim, just based on the listed powers. Many of those powers are Rego-oriented, too, and at least one is Creo-oriented. That doesn't leave much that couldn't be covered.
Aquam and Ignem are really the only two that aren't common to at least one of the Lineages. That is not saying there is not Hermetic Cats that would be appropriate for those two, just that they are the only two in which it is not common amongst some portion of their population.
I would tend to say yes for a few reasons, one of the big ones is the sheer number of alternatives. Fundamentally there are two ways to look at it, which all comes down to the Tremere-
If the Tremere are highly conformist then wolves could account for roughly 4% of all familiars, with literally thousands of animal options, some of which may be regional, cats are likely around 3%.
Or Tremere have about the same distribution of familiars as everyone else, and cats are the most common.
One thing I didn't talk about related to the White Lineage.
They are descended from Mercere's familiar, are commonly found on House Mercere properties, and many of them actually enjoy traveling. The lineage is often well-traveled and the best at combat of the Lineages (per Common V/Q). Among their Common Flaws are combat damage and their "owner" as a Dependent.
This means that there are a fair number of Redcaps traveling around with a White Lineage Hermetic Cat (however much "Many adopt a particular human to travel with" would cover in your individual game). They would provide some additional security on more dangerous routes. What this means is that a White Lineage cat could show up at any Covenant that is visited by the Redcaps.
A Covenant in a remote or dangerous area has a nice potential Story Seed. If the Redcap who visits them travels with a White Lineage, what would it mean if suddenly that cat showed up alone (possibly injured) at their Covenant one day?
