In this sentence, the purpose is to put the slow dog as the subject rather than describing it.
You can tell this because "Heh Hayediah" ("ה"- the equivalent of "the" in English) is used both before the word dog and slow.
If you wanted to describe the dog and say that it is slow you would say:"הכלב איטי" (the dog is slow) or "הכלב הזה איטי" (this dog is slow) and omit the "ה".
I have noticed that in some books that I have and on some translators, some of the hebrew is spaced out differently.
Ie. האיטי actually translates to Haiti, but when the ה is separated from איטי, then it becomes "the slow". This is a horrible example of the distinction as we know that ה in most cases has come to mean "the"...but, also is the case of using ו to mean, "and", I understand what it means, but in some of the books the ו is also separated so that it would look like אמא ו אבא rather than אמא ואבא...
Is this just a personal preference of the authors/programmers?
I've now put this sentence into Google translate, and it produced האיטי, just like DL (same if Just put "slow").
Anyways, without the first yod it's an almost legitimate spelling. When writing with niqqud you don't write the first yod. The standard for niqqud-less spelling calls for the yod, but many people don't know or don't follow the exact standard, and in this particular word I'm not surprised to see it often written אטי.