I had not heard of them eating cats before these recent allegations started cropping up. But I do recall similar allegations and rumors about Vietnamese when they started moving into California, about them catching and eating their neighbors' dogs. People would even joke about things like that, although many viewed it as just a typical racial/ethnic stereotype. There was a time when people said that black people liked watermelon. One could say "Black people are known to eat watermelon" and be factually correct, but not all black people like watermelon.
The issue, as I see it, is not whether or not there might be one or a few Haitians who might have eaten cats. It's quite possible that there are some cases which could be found if one looks hard enough. But that's really beside the point. The issue is, why would Trump bring it up to begin with? He's tarring an entire group of people over what may be a few isolated incidents. It's like he's trying to say "If you don't vote for me, then this country will be overrun by people who want to eat our cats."
It's the old political strategy based on the understanding that people don't vote out of love, they vote out of fear.
But it seems more people fear Trump than the possibility of a Haitian immigrant eating their cats.