That's just an insensitive comment of yours. As if it being just an "animal" somehow means that it was less of a loss or tragedy. Because its "just" an animal should we not compare numbers to numbers for perspective? Isn't that a rather silly stance to have when talking about deaths in the first place?
Humans are an animals too, the most numerous in fact. We should show some empathy and understanding in regards to all life. Not separate and somehow feel indifferent about anything "non-animal".
This is an example of the (probably universal) human bias that some things, like human lives, are sacred. When we see sacred things treated as even able-to-be-compared to mundane things, flouting their sacredness, it provokes a visceral kind of revulsion, like seeing one's religious icons spat on.
Like most biases, this sacredness phenomenon is not all bad; it's an important heuristic allowing us, as it allowed cavepeople, to make complex moral judgments at a moment's notice. But in the modern world, constantly faced with the need to make evolutionarily-strange types of judgments, it's important that we recognize this feeling as what it is: a tool for instantly making reasonably-accurate decisions.
Comparing the scale of horse deaths to the scale of human deaths isn't an attack on the value of human life; if the author had to save a human or a horse in a snap decision, I'm sure they'd save the human without thinking. They are not a sociopath with a broken sense of sacredness, they're someone who has learned to do an unnatural thing: put aside their gut instinct and speak rationally about these important issues.
Horse is an animal. I find it highly disturbing to compare their 'loses' to actual humans.