This is how Pluto costumes changed.
The first costume was on four legs.
The second Pluto costume, seen in the 1960's and 70's, was on two legs, but his mouth was closed, giving him a bit of a mean look.
The next Pluto costume (pictured here with Goofy
and Donald Duck
), in the 1970's, had larger eyes.
The current Pluto costume, first seen in the late 70's/early 80's, has a happier, friendlier appearance, with his mouth open and his tongue sticking out.
This costume, based on the second design (and also the fourth), is used in Saving Mr. Banks