In Russian linguistics, there's a term дифтонгоид (diphthongoid). For example, in textbook Современный русский литературный язык (Modern Standard Russian) by S.V. Knjazev and S.K. Pozharitskaya, it is introduced as a unit opposed to a true diphthong (Knjazev argues that a true one should have two rather equal parts, while in a diphthongoid one part is shorter than the other).
The questions are as follows:
- Is it true that in the western tradition this term isn't employed? (Although see https://en.wiktionary.org/wiki/diphthongoid and https://doi.org/10.1017/S0025100314000395)
- What are the cases like Russian [ᶷo] (see the latter link, page 225) called? Diphthongised [X]?
- Is it correct to say that a diphthong has 'two rather equal parts'? To my mind, it's a continuum from A to B, so, e.g., [aɪ] is a glide starting in somewhat like [a] and ending up in somewhat like [ɪ].