Done using save modification for demonstrative purposes, since otherwise Terapagos will immediately change form upon entering the battle. As you can see, Terastallizing into Stellar with Baby Terapagos doesn't cause a strange delay/freeze effect the same way an illegal Tera Type would, which I showcased previously here: • Baby Terapagos Terastallizing into Ghost -...
See all mechanics tests here: • Pokemon SV Mechanics Testing and Research