The changeling form of Vector Mark. he's actually an older race Changeling. His kind of Changeling looks more insectoid, can sustain a shape much, much longer than "normal" Changelings and without thinking (and the transformation goes so deep they become physically the pony they transformed into up to the bone), they are way less "hungry" for emotional energy, but their transformation is much slower and in their insectoid state they can't talk. Every individual of the Changeling race has no assigned sex, and inherit it from the form they take. If they want to reproduce, they need either another changeling, or a male pony for a partner. Due to an ancient curse, the baby will need a lot of emotional energy to survive. A changeling can provide that, but if a pony would carry a changeling baby, it could be fatal to both the mother and the child.

It was a shock when Vector found out that his wife Mayflower was pregant of him, and even more so that his old self had done this intentionally. It turns out it was part of a prophecy to break the ancient curse, which spoke about a changeling child being born the evening after the day where it was both day and night at the same time. Vector thought this was impossible, but a few months later, to his surprise it had actually happened, and he rushed with his wife to Ponyville to find the only pony he knew who could help him: the newly crowned princess Twilight Sparkle. Together with Twilight's friends, they came up with the ingenious plan to organise a party (thanks, Pinkie Pie!) where Vector could keep on channelling emotional energy to keep Mayflower alive for the birth. She narrowly escapes the ill fate of the curse, and their hybrid daughter Caterpillar was born, but it had also cost Vector greatly, and he transformed back into his natural form, completely drained.

Recovering in the old oak library, he also reveals his secret to his best friend Plus Minus, who's also shocked to discover his friend's true identity. Vector can't talk normally, but Fluttershy is able to interpret his "clicks". He has to get back home, but how would they go about having a scary looking insect travelling on the street. It was Rarity who came up with the idea to get him some clothing to make him look less intimidating, and their plan was working (with the addition of Mayflower convincing a curious little filly that he's an alien from another planet who wants to go back home). On the station he manages to transform into a hybrid form (similar to their daughter), where he's able to talk normally again. With his friends knowing his secret, his wife saved from death, and a newborn daughter in their family, this was another major life-changing day for him.

So yeah, quite some lore behind this creature...

Drawn in InkScape