Algiz is a useful corrective for anyone inclined to think the futhark is well understood. Three separate things about this character are unknown.
The name
The form *algiz, elk, is a modern reconstruction. It is extrapolated from the Old English name eolhx or eolhx-secg, which the Old English Rune Poem describes as a sedge that grows in the fen, wounds and burns anyone who grasps at it.
Eolh does mean elk in Old English. Eolhx-secg means elk-sedge, a plant. The poem is describing the plant, not the animal. Whether the Elder Futhark name was the animal, the plant, or something else again is not recoverable.
The sound
The rune's value in the Elder Futhark was a sound conventionally written z, the reflex of Proto-Germanic *z, which in North Germanic became a sound written R and eventually merged with r. It never occurred at the start of a word, so the rune could not be named by the ordinary acrophonic method, you cannot name a letter after a word beginning with a sound that no word begins with.
This is why the name is unstable, and it is a genuine structural oddity in the row.
The meaning
Modern practice reads Algiz for protection, and for the elk's antlers or a hand raised in warding. Both are readings of the shape, which is a legitimate way to interpret a symbol and is not the same as knowing what it denoted.
The inverted form
The character reversed became a letter in its own right in the Younger Futhark, named yr. That reversed form has a specific modern history: it was adopted in twentieth-century Germany as the Todesrune, the death rune, printed to mark dates of death, and it is still in circulation in far-right symbolism.
Readers who work with reversed runes should know that the reversed Algiz is not simply this atlas's Algiz turned over. It is a historical character with a name of its own and a recent political afterlife.