Yes, sorry shouldn't have used a dragon as an example was just first large creature that came to mind. I understand that it is not tactically sound, playing a one off adventure this weekend and I made tremors out of my eidolon. Thought it would be interesting having a burrowing buddy come out of the sand with tremor sense and swallow enemies. Just trying to figure out all the rules before the game in case the DM has questions.
(as an aside does stoneskin cast on my eidolon also transfer the dr to people trying to slash their way out?)