Sorry to, er, upset you, Butterfree, but there is no "L" in the Japanese language.
v.v Does nobody understand a word I say? Christ, you're the second person I've been trying to explain this to in the past couple of weeks and it seems to be impossible to get through anyone's skull.
The fact there is no L in the Japanese language is precisely the point.
(Well, it is technically inaccurate to say that; the "r" sound in Japanese is actually kind of midway between L and R as we know them, and if a Japanese person says something that sounds like "ra" and you ask them to repeat it, it might as well sound more like "la" when they do. There is just no meaningful difference between the sounds in Japanese. But back to the point.)
The names of Latios and Latias are derived from
Latin, not Japanese.
Latin has an L.
Latin is perfectly capable of just having the syllable "ti" and ending a word in an S.
In order to write these LATIN names, "Latios" and "Latias", in Japanese, the game creators had to approximate them as far as possible using the syllables that exist in Japanese, rendering them as ラティオス and ラティアス.
Those approximations,
without accounting for the fact that they are just approximations of words written with the Latin alphabet to begin with, would be transcribed with our alphabet as "Ratiosu" and "Ratiasu". However,
because the actual intended names are LATIN, not Japanese, and are written with the Latin alphabet as "Latios" and "Latias", those are the official romanizations of the Japanese names.
If you walked up to an employee of Game Freak, showed him a picture of Latios and asked him to write its Japanese name with our alphabet, he would write "Latios", not "Ratiosu".
Let's take a parallel example if you still don't get it. Japanese uses the ENGLISH word "hamburger" as a loan word. Being an ENGLISH word, it is correctly written with the Latin alphabet, as "hamburger"; however, for the purposes of using it in Japanese, it has to be approximated using syllables that exist in Japanese, as ハンバーガー.
Without accounting for the fact that "hamburger" is an English word, originally written with the Latin alphabet, you would transcribe this as "hanbaagaa".
"Hanbaagaa" is just an attempt to write "hamburger" with an alphabet that can't write it properly. If you're writing in the Latin alphabet anyway, and are aware that it's supposed to be "hamburger", why on earth would you insist that the Japanese word for a hamburger is not "hamburger"? Similarly, "Ratiosu" is just an attempt to write "Latios" with an alphabet that can't write it properly, and if one of them is more correct to use, it is "Latios", since
that is the official romanization of the name. Japanese Pokémon merchandise sometimes writes Japanese Pokémon names with the Latin alphabet, and then it will write "Ebiwalar" on a Hitmonchan wallpaper, and "Showers" on a Vaporeon figurine, and yes, "Latios" on a Latios plushie.
I don't know how I can possibly make myself clearer than this.