The phrase "on high horse" typically refers to someone who is acting arrogantly or pompously. The opposite of this would be someone who is humble and modest. You could describe such a person as down-to-earth, unassuming or low-key. Alternatively, you might use terms like approachable, affable, or friendly, which suggest that the person is easy to talk to and not at all stuck-up. It's also possible to use more negative antonyms for "on high horse," such as unpretentious, unassuming, or self-effacing, which imply that the person is lacking in ego or self-importance. Ultimately, the best antonym for "on high horse" depends on the context and tone of the conversation.