...previously Centipede girl thoughts are written in the same Antique-Gothic font as the other text. This 'composite font' is the main font used in most Japanese manga, named so because it comprises both cursive (Antiqua) kana and sans-serif (Gothic) kanji. Nearly all dialogue in Japanese manga is written using this font style, especially if they are found contained within speech bubbles. Out-of-focus text found outside bubbles will typically use a "maru" variant of the same with rounded strokes around the corners (and less brush 'flourishes), which helps aid legibility for smaller text.

Centipede gurl now uses a Mincho ("Ming Dynasty") font for her private "monster" thoughts. This font has little upwards-pointing "triangles" (called "uroko", "fish scale") near the end of 'brush' strokes, which are basically the Japanese (and the rest of the CJKV sphere) equivalent of Western serifs. So, kinda like a Japanese version of serif font, basically.

The specific Mincho variant used appears to be MS Mincho (or some variation), a common Mincho font that comes bundled with most Windows installations (especially on older computers running on XP or older without many other choices to fall back on). This font is also the usual "fallback" font for most browsers when rendering websites that fail to specify "Japanese" properly under the stylesheet.

MS Mincho is infamous for being... well, UGLY (just look at the thin strokes and that... UGH... 'hook' on い). And a bit illegible. The hatred it causes is not too dissimilar to what some English-speaking speakers would think of, say, Comic Sans MS (Oh look, Microsoft again!). But its usage here is... actually quite apropos because it causes a very 'jarring', somewhat 'alien-like' effect when reading Centipede Girl's thoughts. Quite brilliant of Ishiyumi-sensei too, if it's done deliberately.