Help me understand why this hiragana and katakana were combined?

Hi! So I took Japanese for 4 years in highschool like 13 years ago, so I’m incredibly rusty with my Japanese and haven’t really practiced.

I am coming here to ask why this name “Takkun” from FLCL would be spelled “タッくん” in the Japanese version of the manga. Isn’t the “タッ” part katakana? I was taught that katakana was reserved for non-Japanese words. So, I can only assume that the cat’s name is actually “タッ” and the “くん” is to denote he’s a boy cat?

I would have assumed it would be spelled “たくん” but now I’m guessing the cat’s name is Ta?

Thanks for any help!