gdamn I knew I was forgetting someone . . . yeah, that name is extremely retarded . . .
desu / masu are used to end sentences in Japanese's "standard" polite form.
. . . but, in this case, when combined with the design of the pokemon itself, you end up reading it as shortened Katakana.
Katakana words are foreign words, but Japanese will also abbreviate foreign phrases by using only the first segment of each word. Examples include Raji-Kase (radio/cassette player) and Ame-Futo (American Football). They also use this method to nickname bands with western sounding names, like Tora-Nesu (Trap Nest) or BLAST (Black Stones).
So in DesuMasu's case, the katakana word Desu is often used as the closest pronunciation to the word "Death" (hence Yu-Gi-Oh cards like Des Koala, Des Wombat). Where as Masu is used to abbreviate "Mask".
So DesuMasu is Desk Mask, but also puns off of DesuMasu which refers to a form of speech.
. . . yeah . . .
"Hmmm... a name for this plan..."