Arlo has many possible derivations: Spanish for "barberry tree", Arlo was first used in English by Edmund Spenser in The Faerie Queene in the 16th century and it is thought he took the name from the celtic word Aherlow.
Arlo, strongly associated with shaggy singer Arlo Guthrie, has an animated and cheery feel, thanks to its upbeat o ending. Arlo was chosen by Toni Colette for her son, which might give it a further bounce in popularity, while Johnny Knoxville nudged Arlo into the unisex column when he used it for his daughter.
Arlo was popular in the first half of the twentieth century--it was Number 667 in 1915, for example. However, it's been experiencing a bit of a revival and might even surpass its peak in popularity -- it was one of the fastest-rising names of recent years and currently sits at Number 539. Arlo is even more popular abroad, reaching Number 85 in New Zealand and 187 in the UK. Arlo is a greta favourite on Nameberry, sitting at Number 138.