In the Daring Do book series, Dr. Caballeron made his debut in book 4, Daring Do and the Razor of Dreams. He is stated to have wanted to partner up with Daring Do on her adventures, but Daring Do refused.
Caballeron is working with Ahuizotl. He wanted to sell the Ring of Destiny to the creature, make a fortune and retire in splendor. What Ahuizotl intends to do with the ring is apparently of little consequence to him. Caballeron and three stallions take the ring by force from Daring Do. Actually Caballeron doesn't care about who wants the ring as long as he/she pays for it.
In fact in the forest, Dr. Caballeron and his henchponies encounter a disguised Daring Do who offers a large bag of bits for the ring and they have a deal. When Ahuizotl angrily storms onto the scene, Caballeron makes off with the bits and leaves the ring behind.