The local community thrives through tourism that the Atotonilco Hot Springs attracts, so if you ask around, you should be able to find a guide quickly and easily. We used <INSERT NAME + NUMBER HERE>. The younger man spoke conversational English, but the guide spoke next to none.
The going rate in August 2020 was 700 MXN Pesos which is about $33 USD per person. This rate included a local guide for the group and a horse per person.
Neither my girlfriend nor I had any horseback riding experience, but we were able to manage with the instructions given.