This is how the Thumper costumes changed in the parks.
The oldest known costume of Thumper, as early as the 1960s, it appears to be his child form, as distunguished by his single buck tooth.
His next form, presumably from the 1980s to 90s, had his neck frill added, While his feet are grey and have no toes. While hard to tell, it is assumed he had both his buck teeth, therefore he took some steps closer to his adult form.
His latest form is a lot more closer to his animated appearances, and is obviously his adult form, as seen by him having both his buck teeth, and more developed neck frill, his feet now have 3 toes and are white, his eyes now are amber coloured and his front paws have pink pads, although hard to see, he also has tap dancing material under his feet, allowing him to do his thumping.