Kithkin weren't exactly all that popular when they first came out.

Wizards of the Coast tends to work hard on it's various races for Magic: The Gathering. Combining existing mythological creatures, animals, different beings from fiction, and a bunch of other sources, the races of Magic have seemilgly touched on every culture and variant by this point. But sometimes they went a little too far in their references. Or, should we say, they didn't make enough changes. 

Kithkin were introduced in 2007 in the Lorwyn block (Yes, yes, we know about the Legends card Amrou Kithkin, but we'll get to that in a moment). These small elf/human/rat type things had a lot in common certain other halfling creatures, with Magic obviously trying to make their own version of hobbits. And, well, that's where a lot of trouble came from actually.

During the concept stage of Lorwyn, artist Steve Prescott used what the Magic team wanted and created what looks like hobbits gene-spliced with rats. Seriously, if this got past concept stages, there could have been an entire race of these:

However, after a bit of negative reception, they slid those guys back to be more human. In fact, they became hobbits. Literally. They were made short, with hairy feet and, oh yeah, were called hobbits. As fun as this would have been, having Bilbo battle werewolves and hippogriffs, it hit a snag. The estate of J.R.R. Tolkein, the creator of hobbits and the author of the The Hobbit and the The Lord of the Rings series, wasn't too keen to have the race be allowed in Magic: The Gathering form.  Besides, they already existed in a collectible card game: Decipher's The Lord of the Rings trading card game, which ran from 2001-07.

So, in short, WotC needed a new name. Legends has a one-off, hobbit-like creature in it by the name of Amrou Kithkin and WotC decided to go with its last name then and there as the official name for the creature type. As the name comes from old English words for friends and family - kith and kin, respectively - it was all rather fitting. Features were also changed a bit, going from hobbits to more of a gnome/dwarf/halfling/elf/human sort of mix with almost all rat-ness finally taken out.

However, despite finding the right equilibrium name-wise, player response was a whole other thing. They simply weren't all that well liked. Mark Rosewater said that market research found that players saw them as creepy, and they quickly fell out of favor. No more kithkin were printed past Eventide the very next year, and they have since rarely made any more appearances.

Kithkin were supposed to be an amazing new race, but major creature type overhauls, legal issues with the Tolkein estate, names coming up on the fly, and an overall creepy appearance more or less doomed it. What could have been a unique race quickly devolved into compromises.