"Stop! Hibarikun (1983)" involves a trap, and although it does not have any transformation, but it predated Ranma 1/2 so I'll mention it since it influenced the TSF genre. EM003 uses Hibari-kun as their mascot, and you should try reading the series at least once to see how much the genre has changed in 40 years. (I don't actually like the OP or ED though.)

https://youtube.com/watch?v=OUxbg1_r_CU [Embed]