The Fathomless Warlock has a feature called Tentacle of the Deeps, which allows them summon a 10-foot long spectral tentacle (exact wording below). It can be moved and make attacks,but is spectral and doesn't necessarily have a physical presence. However, it can affect events around it by dealing damage, partially blocking both physical and magical damage types at later levels (described in Guardian Coil below), and moving around. This raises the question of if it is considered a creature for the purpose of flanking. While it's never explicitly stated to be one, it shares many traits of one, and as evidenced by many creatures like ghosts, being corporeal isn't necessarily a requirement to be considered a creature.
Tentacle of the Deeps:
At 1st level, you can magically summon a spectral tentacle that strikes at your foes. As a bonus action, you create a 10-foot-long tentacle at a point you can see within 60 feet of you. The tentacle lasts for 1 minute or until you use this feature to create another tentacle.
When you create the tentacle, you can make a melee spell attack against one creature within 10 feet of it. On a hit, the target takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn. When you reach 10th level in this class, the damage increases to 2d8.
As a bonus action on your turn, you can move the tentacle up to 30 feet and repeat the attack .
You can summon the tentacle a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.
Guardian Coil:
At 6th level, your Tentacle of the Deeps can defend you and others, interposing itself between them and harm. When you or a creature you can see takes damage while within 10 feet of the tentacle, you can use your reaction to choose one of those creatures and reduce the damage to that creature by 1d8. When you reach 10th level in this class, the damage reduced by the tentacle increases to 2d8.
Best Answer
No, it would not help you with flanking.
Flanking requires status as a creature, as you mentioned, and nothing about the two abilities linked mention that this is a creature.
We can use this Answer for an example of 'what is a creature'. The ability does not hit at any of these.
The Tentacle is not an independent, or controlled, entity. It has no health, armor class, statistics. The tentacle isn't even the actual being to make the attacks that it makes, you are mechanically making spell attacks against anything within 10 feet of the tentacle it isn't actually 'making the hit' you are. The tentacle lacks any ability to take actions, reactions, move itself, and other features that you would see on a creature for 5e.
The issue of it being 'spectral' isn't a problem. A ghost is incorporeal but a ghost also has everything I just mentioned. Things like Bigby's hand, Spiritual weapon, and other spells also do not create creatures.