What's the glyph origin of 妖?
《說文解字》沒有「妖」這一條目。
There is not an entry of 妖 in the 《說文解字》.

《說文解字注》對「」的解釋有提到「俗省作妖」。
The interpretation of  in the 《說文解字注》 mentions 俗省作妖.



 [ yāo | ㄧㄠ ]


意思是，民間習慣上，將「」省略為「妖」。
也就是，「妖」的本字是「」。
It means that  is used to be simplified to 妖.
That is, the original form of 妖 is .

《說文解字》對「」的解釋如下：
《說文解字》 explains  as follows.




  ，巧也；一曰，女子笑皃。


皃 [ mào | ㄇㄠˋ ] 同「貌」。


所以，「」有兩個意思：一是「巧」，二是「女子笑的樣子」。
這應該也是「妖」的本義。
Therefore,  has two meanings: "skillful or clever" and "the appearance of a female smile".
This should also be the original meaning of 妖.

至於，「妖」會有「邪異」的意思，是來自「祅」字。
As for the "demon" meaning of 妖, it comes from 祅.

同樣的，「祅」的本字是「䄏」。
Similarly, the original word of 祅 is 䄏.

祅、䄏 [ yāo | ㄧㄠ ]


《說文解字》對「䄏」的解釋如下：
《說文解字》 explains 䄏 as follows.




  地反物爲䄏也。


「䄏」是地上反常的物種。
䄏 is the abnormal species on the ground.

《說文解字注》在解釋此句時有提到「經傳通作妖」。
The interpretation of this sentence in the 《說文解字注》 mentions 經傳通作妖.



《說文通訓定聲》說這是一種假借。
（妖、、祅、䄏都是形聲字。）
《說文通訓定聲》 explains that this is a loan.
(妖, , 祅 and 䄏 are the picto-phonetic characters.)



Conclusion:

「妖」的本義有兩個意思：一是「巧」，二是「女子笑的樣子」。
被「祅」假借，所以有「邪異」的意思。
 has two original meanings: "skillful or clever" and "the appearance of a female smile".
It is borrowed by 祅, thus it also means "the demon".



Is there a website to look up the evolution of characters?

I recommend 小學堂-字形演變 to look up the evolution of characters.

小學：研究文字字形、字義及字音的學問，包括文字學、聲韻學及訓詁學等。
小學 does not mean the primary or elementary school here. It means the study of the glyph, meaning and the pronunciation of the word, including philology, phonology and exegesis.

(Input the character in the 字形 field.)

For example, 中.