Mentioned Enemies are enemies that are mentioned in the Pikmin series, but are not seen as actual enemies. 

Burrowing Snarrow

Burrowing snarrow

The supposed "Burrowing Snarrow", but could be a Beta Burrowing Snagret.

The Burrowing Snarrow is a relative of the Snagrets. Olimar notes that the Burrowing Snarrow can be distinguished from the Burrowing Snagret by the absence or presence of tail and wing markings. Also, the Burrowing Snagret's feathers are blue, while the snarrow's feathers are not. This creature is mentioned in the Enemy Reel of Pikmin
The bluish hue of its feathers distinguishes it from the Burrowing Snarrow.

And once again in the Piklopedia of Pikmin 2:

[...] Visually resembling the Burrowing Snagret is the Burrowing Snarrow, the range of which partially overlaps with the snagret's range. While the two may appear similar, when pulled from the ground they can be distinguished by the presence or absence of tail and wing markings.

According to Olimar's notes in Pikmin 2, the Burrowing Snarrow had the ability to fly, as in Pikmin 3, the snagret is revealed to have a foot. In a Beta trailer of Pikmin, there is a Snagret with a different hue, suggesting it could've been the Burrowing Snarrow. However, it could as well have been a Beta Burrowing Snagret. 

Pond Skater

Not to be confused with the Skeeterskate, a pond skating enemy from Pikmin 3.

The Pond Skater is a evolutionary ancestor of the Skitter Leaf. It is stated that it had wings and could fly, as well as skate on water. When Pond Skaters adapted to life on dry land, they lost the ability to skate on water, and their wings turned into a leaf-like structure, which acts as a form of camouflage. These were the first Skitter Leaves. They are mentioned briefly in the Piklopedia of Pikmin 2: 

"The skitter leaf is a relative of the pond skater that shed its wings and adapted to life on the ground. With no residual traits of its airborne past, the skitter leaf can neither fly nor skit across the surface of the water."

Female Shearwig

Not to be confused with regular Shearwigs.

While Shearwigs are considered an enemy, only their male counterparts appear. The females supposedly live most of their lives underground, but come up for brief periods to breed. Male Shearwigs evolved the ability to fly, but females (considering they live underground) did not. Female Shearwings are mentioned here in the Piklopedia:

"However, only the adult males of this species can fly. Females of this species spend most of their life span underground. They do emerge for a period after maturation to spawn, but never metamorphose."