When Hermione first got her pet thing Crookshanks, there was a mention that it looked kinda like a cat, but wasn't.
Was it ever revealed what Crookshanks really was?
Yep; Crookshanks is half-Kneazle.
From JK Rowlings site:
Crookshanks, as anybody who has read Fantastic Beasts and Where to Find Them will have guessed, is half Kneazle. And if you don't know what a Kneazle is, you need to hurry up and buy Fantastic Beasts and Where to Find Them (all royalties go to help some of the poorest children in the world).
You might also want to check the Crookshanks page on the Harry Potter Wiki.
J.K. Rowling first hinted this in her description of a Kneazle in Fantastic Beasts and Where to Find Them and in a comment on her old website.
Kneazle
M.O.M. Classification: XXXThe Kneazle was originally bred in Britain, though it is now exported worldwide. A small catlike creature with flecked, speckled, or spotted fur, outsize ears, and a tail like a lion’s, the Kneazle is intelligent, independent, and occasionally aggressive, though if it takes a liking to a witch or wizard, it makes an excellent pet. The Kneazle has an uncanny ability to detect unsavoury or suspicious characters and can be relied upon to guide its owner safely home if they are lost. Kneazles have up to eight kittens in a litter and can interbreed with cats. Licences are required for ownership as (like Crups and Fwoopers) Kneazles are sufficiently unusual in appearance to attract Muggle interest.
(Fantastic Beasts and Where to Find Them)
Neither of these characters can perform magic (Filch’s Kwikspell course never worked), but they still function within the wizarding world because they have access to certain magical objects and creatures that can help them (Arabella Figg does a roaring trade in cross-bred cats and Kneazles, and if you don‘t know what a Kneazle is yet, shame on you).
(JK Rowling's old website - Extra Stuff - Miscellaneous - Squibs)
She also stated this more directly on her website and then later on twitter.
Crookshanks, as anybody who has read Fantastic Beasts and Where to Find Them will have guessed, is half Kneazle. And if you don't know what a Kneazle is, you need to hurry up and buy Fantastic Beasts and Where to Find Them (all royalties go to help some of the poorest children in the world).
(JK Rowling's old website - Extra Stuff - Characters - Squibs)
Spooky Magdalen: who would win in a fight, Mrs. Norris or Crookshanks?
J.K. Rowling: It would be brutal and very close, but Crookshanks' Kneazle ancestry would bring him out on top.
And not that the current Pottermore has any more authenticity than the wiki, but they confirm this in their fact file on Crookshanks.
Hermione’s sharp-witted pet cat, who is half-Kneazle