Very cute transformation drawing :) I especially like the not-quite-fitting panties :)

Translation of description:

ポニー化すると首筋が弱くなるらしい
“It seems that when you turn into a pony, your neck becomes weak.”

ポニー = Pony
化する (ka suru) = to change into
と = is a conjunction

首筋 = the nape of the neck
が = as-for or “in specific”
弱く (yowaku) = weak
なる (naru) = to become (similar to する above except that する (to do) turns a noun into a verb. So in “化する” (ka suru), “化” (ka) means “change” or “transformation” and “化する” means “to transform” or “to change into.” It indicates the action of changing from one state or form into another. なる (to become) on the other hand indicates that the adverb or adverbial has become a ‘state of being’. “なる” is used when something changes state more passively or naturally, like “The sky becomes red at sunset” in 夕日で空が赤くなる.)

So - all together, 首筋が弱くなる means “the nape of the neck in particular has become weak”.

らしい (rashii) just means “it appears”. It’s conjecture. So the sentence “ポニー化すると首筋が弱くなるらしい” is more of a “Huh. I guess this happens then” kind of statement.

Removing the “らしい” from the end of the sentence would make it more active and definitive. “ポニー化すると首筋が弱くなる” is just “Transforming into a pony makes your neck weak.”

Substituting the adverb “必ず” (kanarazu) for “らしい” would make this much more definitive. However, it gets inserted into the sentence before the object of the verb - “ポニー化すると必ず首筋が弱くなる”. As in: “When turning into a pony” + “WITHOUT FAIL” + “the neck becomes weak”.

Adding らしい back to create “ポニー化すると必ず首筋が弱くなるらしい” ads an interesting nuance. It’s a way of saying that it definitely happens - apparently …

Personally, since I usually was working with superiors or customers, I’d probably have added “でしょうかね” (deshyo ka ne?) instead. This is a polite form of でしょう (the infamous desu-form) that indicates conjecture, that adds か (ka) the ‘this is a question’ marker, and ね (ne), the particle that seeks agreement or confirmation. It’s like adding “Don’t you think?” to the end of an English sentence.

That way you’re inviting the listener to confirm what you’ve said.

Be careful how you use でしょうかね though - it’s the kind of thing that teachers use to put students in their place. As in “That was rude, WOULDN’T YOU AGREE?”. Syntactically, in Japanese, the student almost always has to agree with the teacher to compose any response.

So, use sparingly and in a self-deprecating way, or you can come across a bit like a bully.

変化したてで尻尾ブンブン振ってるシチュが好きです
“I like the situation where the tail is wagging excitedly right after the transformation.”

変化したて is “just transformed”, built from 変化 (Henka) which means to change, and したて (shitate) which means that something is fresh or just happened.

で (de) is your trusty do-everything-but-the-dishes particle that marks the context.

尻尾 (shippo) is just “tail”. Surprising to find such a simple single meaning thing in the midst of all this, isn’t it?

ブンブン振ってる (bunbun futteru) is “frantically wagging tail”. ブンブン (bunbun) is onomatopoeia for vigorous movement, like a car swaying down a road or a tail wagging like crazy. 振ってる (futteru) is the contraction of 振っている (futte iru) which is the verb for “swinging/wagging.” 振って (futte) is the te-form of the verb 振る (furu), which means “to wave” or “to swing”. いる (iru) is like “ing” in English - it’s a “helper verb” used to indicate that the verb is “ongoing”. So, you don’t just “swing the tail once” - the swinging continues.

シチュ (shichu) is the short form of “シチュエーション” (shichueshon). It’s exactly what it looks like - the English word “situation” written in Katakana.

が (ga) marks the subject specifically. Not just “as-for” form, but “this, specifically” form.

好きです (suki desu) means you like it. The desu-form just makes it polite. You could be more polite, but it’s pretty polite as it is.

So, this sentence is a polite form of “I like the situation where the tail is wagging excitedly right after the transformation.”