ẗ is a modified letter of the Latin alphabet, derived from the letter T with a diaeresis on it. It is used in the ISO 233 transliteration of Arabic to represent tāʼ marbūṭa (ﺓ, ﺔ), and also in the Uralic Phonetic Alphabet to represent a tenuis interdental stop [t̪͆].
Only the minuscule form exists in Unicode as a distinct character. The majuscule must be formed with a combination of T and a combining diacritic (T̈), and because of this may not display correctly when using some fonts or systems.
References
T-diaeresis Wikipedia(Text) CC BY-SA