right|thumb|A partially reconstructed temple of Apollo Grannus at Faimingen (Phoebiana) near [[Lauingen]]
Grannus (also Granus, Mogounus, ("The one with a piercing or far-reaching look"), and at Horbourg-Wihr Apollo Grannus Mogounus.
In all of his centres of worship where he is assimilated to a Roman god, Grannus was identified with Apollo, The town's hot springs with temperatures between 45 °C and 75 °C lay in the somewhat inhospitably marshy area around Aachen's basin-shaped valley region. Caracalla's visit to the shrine of ‘the Celtic healing-god’ Grannus was during the war with Germany in 213.
Festival
A 1st century AD Latin inscription from a public fountain in Limoges mentions a Gaulish ten-night festival of Grannus (lightly Latinized as decamnoctiacis Granni):
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Translation: "The vergobretus Postumus son of Dumnorix gave from his own money the Aqua Martia ("Water of Martius [or Mars]", an aqueduct) for the ten-night festival of Grannus".
Divine entourage
thumb|Altar to Apollon Grannus and [[Sirona (goddess)|Sirona, Baumberg (Germany)]]
The name Grannus is sometimes accompanied by those of other deities in the inscriptions. In Augsburg, he is found with both Diana and Sirona; he is again invoked with Sirona at Rome, Bitburg, Baumberg, and Sarmizegetusa (twice). At Ennetach he is with Nymphs, at Faimingen with Hygieia and the Mother of the Gods, and at Grand with Sol.