Slug Buddy in The Way of the Sponge appears to be red in color. In Squid Defense, it is now mainly brown in color and it appears to be bigger in size as well. The words "Slug Buddy" is also written near the top.
"The Great Snail Race"
Slug Buddy was first used as punching bag by Gary punching it with his eyes.
"The Way of the Sponge"
Slug Buddy was used by SpongeBob when he was training hoping to impress Fuzzy Acorns. Slug Buddy ends up knocking SpongeBob away however.
Slug Buddy was used mainly by SpongeBob and Sandy to help train Squidward for karate. At first, Squidward doesn't even budge Slug Buddy but later, manages to knock it away.
Between the two episodes, Slug Buddy has appeared at SpongeBob's House and Sandy's Treedome. It is unknown if SpongeBob moved Slug Buddy to Sandy's place or they both have different Slug Buddys.