Eevee has been stated by its creator to not being meant to resemble any specific animal to reflect its evolutionary potential, so discussions about what it (and by extension, its equally indeterminate evolutions) is are inevitably going to be rather moot. Still, it's a fun conversation to have, so let's break it down:

For me personally, the Eevee family leans a bit more on foxes than any other animal they share traits with due to that seeming to be the most consistent basis between all of them. That said, I still am reluctant to count the Eevee line as being "true" foxes because they're all still significantly less foxlike in appearance than all the other fox-based Pokemon in the franchise: notably, the Eevee family lacks the distinctive long and pointy muzzle of a fox, with Jolteon being the only one to posses it while the rest instead have a shorter catlike snout. There's also the fact that Eevee and most of its evolved forms posses noticeably long, rabbitlike ears, with Espeon (who posseses ears like those of a fennec fox) and Vaporeon (who has fins functioning as ears) as the sole exceptions. And then there's those which have attributes completely unique to them such as Jolteon's porcupine-like quills, Vaporeon having fishlike traits and a noticeably cetacean tail, Umbreon's rings being bioluminiscent, Leafeon being partially a plant, Glaceon having what's either a natural ushanka cap or Yukki Onna hair, and Sylveon having feelers (which one could interpret as being highly modified whiskers) that resemble bows and ribbons.

Eevee itself in particular to me kind of looks like what we'd get if it was possible for foxes and rabbits to hybridize (as this Zootopia fanart helpfully demonstrates), so for the most part I tend to think of it as a "foxrabbit" thing, though I do occasionally see dog and cat traits in it and its evolved forms as well, to varying degrees. Yes, even the overtly catlike Espeon I see as having a few dog traits (though this one's admittedly mostly due to its Pokedex entries describing it as extremely loyal to its trainer, which is a trait generally more stereotypical to dogs than cats, but hey).

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