Trixie’s changing horn design.
 
Season 1: she has a really long and thick horn that’s distinct from any other unicorn horn seen in the show at that point.
 
Season 3: Her horn is the same length, but it’s thinner to match the horn redesigns that happened mid-season 1.
 
Season 6, No Second Prances: She keeps jumping between her season 1 and 3 models for some reason.
 
Season 6, To Where and Back Again: Her two old models are replaced with a completely new one. Along with two eye reflections instead of just the one she had before (a feature distinctly for male ponies before), her horn is now SHORTER in length. While before it would extend past the top of her mane, now it just barely meets the edge. This has been her model since, used again in Celestial Advice and All Bottled Up.
 
Her model for the movie seems to draw from her late season 6 model, but it’s hard to tell.