It was kind of cool Eragon started to mentally link to him. Selling him was sad. Maybe won't happen in my version.
According to the IC wiki:
Cadoc was the name of Eragon's first horse. Eragon used Cadoc during his travels from Therinsford until Brom's death outside Dras-Leona.
He was a light bay horse that Brom purchased for Eragon from Haberth in Therinsford. Eragon named the animal after his grandfather. Cadoc was the first creature that Eragon touched his mind to other than Saphira. He noted that it was more difficult to speak to Cadoc as he was not as intelligent as Saphira.
After Brom's death, Eragon sold Cadoc and took Snowfire as his own. Even though he was reluctant to part with Cadoc, having traveled almost half of Alagaësia together, he was still determined to fulfill Brom's promise to Haberth that Snowfire would be taken care of. Eragon never reunited with Cadoc afterwards.