The Pluto costume has changed a bit over the years.

Image Year Description

Ice capades 02

Pluto 1955 disneyland ice capades costume
1950-1955 The earliest appearance of Pluto was at the Ice Capades of the early 1950s, when Disney characters were featured prominently in several Ice Capades shows. Pluto is unique among the costumed Disney characters in that it was a two-performer costume, where one person operated the head and the other the rear (like a horse costume). As Walt Disney did not have his own costumed characters at the time, he borrowed the characters, including the two-person Pluto, for use in the opening of Disneyland in 1955.
19601977 Pluto received a dramatic redesign, to make it more park-friendly. He now walked on two legs (thus eliminating the need for two people), had no collar, a closed mouth, a shiny nose and whiskers. There were a few different variants of this Pluto. For example, his pupils changed size, depending on whatever year it was (either big or small, looking here, looking there). His paws also varied: sometimes they were black, sometimes they had individual circles. His ears were also very long.
Disney on Parade 1973
1973 This Pluto (pictured here with Goofy and Donald Duck), seen in Disney on Parade in 1973, had larger eyes (with eyeliner around them at times) and a larger snout with more defined ridges. His ears were not as long as they once were. An orange collar was added, which would eventually show up in the parks.
1977 Pluto had changed little from his 1960 design, but he was given the addition of huge eyebrows.
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1978–today Pluto was drastically redesigned yet again in 1978. This Pluto costume looks more like his cartoon counterpart. He received a resculpted head and new body. He has a happier, friendlier appearance, with his mouth open and his tongue sticking out. He also wears an orange collar with a name tag. Originally, his pupils were much larger than previously. This Pluto only appeared at Disneyland, with a few appearances at Tokyo Disneyland. In Walt Disney World, Pluto had smaller pupils. This particular Pluto was also used occasionally at Disneyland, off and on with the bigger-pupiled Pluto, which was formally retired in 1990 (except for a surprise return for the Disney Sing Along Songs: The Twelve Days of Christmas video in 1993), thus making the smaller-pupiled Pluto the official Pluto in the parks. Only one change was made to the costume since then: in 2005, Pluto's tongue was reduced, not hanging out as far.
Pluto in SMB
2013 This costume, based on both the 1960 and 1978 designs, is used in Saving Mr. Banks.
2016–present Pluto receives an articulated head with a pink cartoony tongue, debuting in Mickey's Most Merriest Celebration. Albeit modeled after the 1978 non-articulated version, his eyes are a bit thinner and still not conjoined to make him resemble his 1960 counterpart.