Foreplay

A random OC which to have 'tamed' Fluttershy, a scene then taken of which the tail is used to display affection. The idea hits into a more romantic/seductive-nature if taken this way and that can be displayed well by Fluttershy's blush and the softening-spread of the OC's tail. Simple but meets the vision and the 'more tails'.

Not particularly original, although it's a rare and nice sight to see an animal-only trait (tails in this instance) actively used instead of placed. The backgrounds rather simple too but it could have still supported the drawing better with a mood-esque color or a mixture as it makes Fluttershy stand out currently as well does not assist when the drawing is attracting a certain mood/atmosphere to it. Overall it's a nice drawing but isn't particularly 'impactful' due to simplicity.

You're drawing style is very pleasant, you added some realism through your style of shading especially to the mane and tail without causing it to 'detach' too much from the smooth bodies that these little ponies have. Approach towards ear-style, working with a relatively line-less style, etc. so Technique stands well.

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Alright, normally I pass on by a drawing that generally does well, but you asked for a critique so here's my analysis.

Fluttershy's cutie-mark is in-accurate, it's one butterfly on top and two on the bottom as well tilted to the sides.

Fluttershy's tail starts to feel stiff near the end because it has the appearance of clumping up strongly due to the addition of strong lighting/shading.

Fluttershy's ear is placed too low, this seems to be the only instance I've seen you draw the ear mid-way of the head instead of at the 'corner' in a sense.

Fluttershy's far eye touches the side of her head which normally when drawing for angled heads you'd still keep a distance from the side of the head to the far eye except when far angled.

Fluttershy's highlights aren't relative to the eye-size, they appear the same for both despite the far eye appearing smaller.

Fluttershy's eyelashes feel too minimal, and personally I see the top-line of the eyes too thin especially on the far eye.

Fluttershy's front hooves are obscured a bit, although the tail works as such it's best to display at the very least move the OC's tail so it cuts off right where the intersection of lines of the two from hooves are. (Best to avoid line-intersections like this in general if possible).

By perspective, the ground appears to curve upward farther from the viewer as the plane at which Fluttershy stands appears as if the viewer is near her position but moreso above the OC because the OC's standing plane is more curved up.

The OC's snout line/dot kinda stands out.

The OC's head seems turned a bit more than naturally comfortable especially because the shading and positioning of the character implies farther than a 90 degree head turn which normally you wouldn't go so far in a calm situation like so.

The shading under the head also displaces Fluttershy's head a bit because normally for these ponies, the side of their heads under the ear connects smoothly to the neck. You have it correct in the 'Rock, Paper, Scissors' drawing with Rarity for example but this may be argued towards a style/preference.

The OC's tail spread is a bit strange because the tail's picked up at the top but from long (left) to short (right) portions of the tail with an entire section that hangs loosely. It'd probably seem more natural to have the whole tail in momentum or to have the tail pick up a certain distance from the hind instead of a 'diagonal slice' of it.

The OC's tail angle/positioning is odd since, based on character positioning, it's aiming straight left. Tail also lacks depth unlike the rest of the mane/tails in the drawing because you applied the same lighting/shading consistently combined with the above making for an illusion at which the tail appears to be behind the OC as normal but is aimed almost perpendicularly left instead when analyzing Fluttershy.

The shading gets strong in the mane/tail but does not delve to be as strong on the body, as well the mane/tail does not seem to cast a shadow on it's surrounding except near the 'forehead' area. Especially noticeable for the OC's back-part of the mane which displays dark shading, but since it's placed 'above' the head that there would be expected of shading on the head there as well.

Artwork composition could use a touch of work because the characters are centered to show focus on them, but they aren't entirely focused on due to having their full bodies visible which again is not assisted by having a simple background. The simple background takes up a good amount of space as well and this 'blank space' is unnecessary for this drawing.

If I may add a suggestion, the eyes kind of stand out a bit due to retaining the show's simplistic style yet you've delved into your own style for everything else, perhaps softening the colors and highlights would help a bit.

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Sorry for the wordiness there, if there's anything really confusing, or you don't understand one of my points, just mention and I'll see if I can explain myself better or to help me see something which I may have missed. I'm a bit confused with my own wording as well sometimes anyways.

Also since you asked for the critique, I may or may not have decided that it was best to analyze and compare to high standards, kinda the way I self-critique myself in fact.