There are no known creatures of this type. Please see Xag-ya.
There is also the glimmerskin. In effect, it is only a parasite, since the glimmerskin uses its slam attack to invade a host's body. Though, it empowers the host creature with pure positive energy, only a strong willed host can function outside of the glimmerskin's control.
The vivacious creature template is another option. It can be applied to any base creature, with the Planar Handbook's example being a dire tiger. The major difference would be the transformation to incorpreal subtype and the added innate benefits of positive energy.
None of these are technically elementals, however.
Sources:
Monster Manual II
Planar Handbook
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