Horen Laal had a C-Rank mana core that he had cultivated within him inadvertently as he kept progressing further on his Path as a Body Cultivator.
However, his mana core was not an accurate indicator of his capabilities. He was a Diamond Body Cultivator who could go against an A-Ranked entity and win without breaking a sweat.
Horen's Laal's acquaintance, Vasco Joel was an equally qualified candidate that had still remained in the race to become a bonafide Demon Slayer. He had high aspirations to become a Demon Slayer so that he could get a better Tantra and compatible element-rich environment for his needs.