According to the unofficial net-encyclopaedia of the games ICO and Shadow of the Colossus, the 16th colossus of the latter game, whose role is also that of the final boss, had two prototype names prior to the final design, those being Evis and Buddha.
Furthermore it lists the names of the colossi that did not manage to get into the final product, an amount of which are only a fraction of the originally planned 48 colossi that due to technical restraints was reduced to the sixteen. Nevertheless, these colossi overlap with their predecessors as some design elements were either borrowed or overhauled to create new ones.
Evis and Buddha are both names for the final 16th colossus and his design was initially that of a monstrous quadruped, but was later in the second and third iterations of the boss changed into that of the towering humanoid. The first incarnation is for all intents and purposes its own entity.
In the official guide to the game, Wanda to Kyozou koushikikouryaku & setteihon Inishie no Chikitan (ワンダと巨像 公式攻略&設定本 古えの地綺譚, "Wander and the Colossus - Official Strategy Guide and Creation Book - The Land of Yore - A Beautiful Tale"), a sketch of this first design is merely glossed with the caption,
"Here is a sketch of what we thought the last boss might be, the planned design"
(p. 192, Inishie no Chikitan)
We are not given any specific name here, merely a confirmation of its status. It does confirm though how drastic this design is to the final one, whose name and stature completely changed.
Evis however is propurtedly from the name of an unused texture that was to be used in relation to the 16th colossus.
Evis itself may ultimately be a typo for Elvis as in the name of the King of Rock and Roll, Mr. Presley, whose forename was given as a developer nickname to the sixteenth colossus during development according to the guidebook, which was also referenced by Glitter Berri's now archived fansite to the game.
This Colossus was called Evis. Because the game is focused on opposing a colossal force, we joked about it being anti-establishment. When you think “anti-establishment”, you think “rock”. And the god of rock is none other than… well, that was my train of thought when I spoofed a certain musician’s name. *laughing*
(GlitterBerri's site, translation of the portion on the pages 141 - 142 that detail the developers' nicknames for the colossi.)
For the sake of comparison here is the original quote from the guidebook:
Îbisu to yondemashista. Honsaku wa kyoudai na chikara ni tachimukatte iku gêmu de hantaiseiteki da ne toiu wadai ga dete. Hantaisei to ieba rokku. Sono kamisama ...... to kangae, aru myûjishan no namae o mojirimashita.
The translation is again the same as Glitter Berri made, one detail that is worth mentionin is that the name is Îbsu, i.e. with an elongated initial "i" sound, which sounds faintly like Elvis in Japanese, i.e. Eruvisu (エルヴィス) - though phonetically a rendering like Irubisu (イルビス), would be a bit closer to the name of the musician himself. Hence the rendering of it as "Evis", and also its name within the unused data of the game itself. Alternatetively it may just be the English words "evil" (イービル, îbiru) and Elvis (エルヴィス, Eruvisu) combined to create the anti-establishment pun/reference/joke that they mentioned in the guidebook.
The God of Rock and Roll, indeed!