Pak E. Derm
Pak Elephant Derm is an elephant-like creature that first appears in Yoshi's Story. Their shortened name, Pak E. Derm, is a pun on the word pachyderm, a term which is no longer in use; it was a taxonomic order of animals in biology that included elephants, whence Pak E. Derms' appearance was inspired.
In Yoshi's Story, this little elephant holds up his stop sign and won't let Yoshi pass. To pass, Yoshi has to Ground Pound the ground, and the Pak E. Derm would fall away, letting Yoshi pass. Yoshi had to act quickly, because the Pak E. Derm would get up soon. Yoshi encounters Pak E. Derm three times throughout the first level. After that, he's absent in all the other levels. Pak E. Derm's Message Block stated the poem that is thought of synonymously with the character: "Pak E. Derm will not let you by, unless you give something special a try."
Super Smash Bros. series
Super Smash Bros. Melee
Pak E. Derm also appears in Super Smash Bros. Melee as a trophy and in the background of the Yoshi Story stage. According to the trophy, there is only one; also according to the trophy, he's been practicing the skill of holding the sign for years. It is unknown why he would take so long, but it could refer to him having no arms and being able to hold the sign anyways, or a joke to give him a more in-depth explanation for his trophy other than something simpler.
Super Smash Bros. Brawl
Trophy information from Super Smash Bros. Melee
Sticker information from Super Smash Bros. Brawl
Names in other languages