@Cuccoruler Looking at the earth dragon description, It is allowed and they may be like the royal guard for your royal dragon. However they may not be royal therefore cannot polymorph/transform into a human. therefore if you royal dragon wants to move around in disguise, she has to leave them behind or it would be suspicious.
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Depends on why you need them? if a mount, there are other creatures that serve better as a mount in my opinion. If pack mole, Why not traditional horse and carriage? etc