From the Super Mario Wiki
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.
Yoshi downs Pak E. Derm with a ground pound in Yoshi's Story
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
Pak E. Derm is one of the many characters to appear as a sticker in Super Smash Bros. Brawl. Information for the sticker can be found below.
Trophy information from Super Smash Bros. Melee
| Pak E. Derm
|| Yoshi's Story
| This pesky pachyderm bars Yoshi's path by sticking poles high into the air. Pak E. Derm has been practicing this skill for years, and he's so masterful that he can even block Yoshi's high-flying Flutter Jump. Pak E. Derm is a little unstable, however, and he tends to topple over if Yoshi pounds the ground anywhere nearby.
Sticker information from Super Smash Bros. Brawl
||[Tail] - Attack +9|
Usable by: Yoshi
Names in other languages
|From 「象さん」 zousan, meaning elephant.
||Pak I. Dermo
||From paquidermo, meaning pachyderm.
||Pash I. Derm
||From pachyderme, meaning pachyderm.
||From tröten (to trumpet)
||Pak I. Derm
||from pachiderma, pachyderm