Someone who buys up old horses for slaughter WordNet 3.0 & Open English WordNet

A knacker, knackerman or knacker man is a person who removes and clears animal carcasses from private farms or public highways and renders the collected carcasses into by-products such as fats, tallow, glue, gelatin, bone meal, bone char, sal ammoniac, soap, bleach and animal feed. Wikipedia

person who removes animal carcasses and roadkill Wikidata

One who slaughters and (especially) renders worn-out livestock (especially horses) and sells their flesh, bones and hides. Wiktionary