「奇」 is not a person riding a horse - that is probably a learning mnemonic, meant to remind you of the word 「騎」. This character has received some attention in the question What is the glyph origin of '奇'?.
「怪」 (Zhengzhang OC: /*kruːds/, strange) is comprised of semantic 「心・忄」 (heart, mind) and phonetic 「圣」(/*kʰuːd/).
「圣」 (Mandarin Pinyin: kū, to dig) is ...
Tl;dr: Isolated use is largely impermissible (points 3. and 4.), because they are classical Chinese. Evidence for which is outlined in point 2.
With other excellent answers detailing the glyph origin of 奇 and 怪, I shall adopt another approach at your question.
On 奇怪 as a compound word
Compound words (複合詞) are rarer in classical than modern Chinese, but ...
It is always good practice to look up the etymology of an idiom whenever you are learning one:
Ruan Ji enjoys wine and a good wail; he is also skilled in playing the guqin. Whenever he derives joy (from these), without warning, he forgets his physical form.
Ruan Ji was one of the Seven Sages of the Bamboo Grove ...
奇 is a controversial one.
There are sites that say it is a person on a horse being original form of 骑, but there are no old glyphs with the horse character in the ancient script.
Easiest interpretation would be large 大 and gasping 可 (mouth + breath). A person is gasping in awe at a large peculiar object. 可 can also double as phonetic, although they both don'...
The meaning lies in 意忘形
Both 意 and 忘 contain 心, the mind
意：音：sound, person speaking + 心 mind = thoughts, ideas
忘：亡：gone + 心 mind = forgotten
形：form, shape, appearance
意忘形: mind forget its form
得：just for tacking this onto other words
她得意忘形, 不要得意忘形, 就会得意忘形
之 was chosen to avoid repeating 的: “每个人都有自己的人生的路，so if you understand 的, you understand 之。
You can often leave out 之 、的：那是人生中(的)一个标记，人生路上的里程碑。
Everyone has their own path in life,
you just need to seize the (right) moment,
(and) strive diligently,
only people who are unafraid of hardships can walk this path
之: marks preceding phrase as modifier of following phrase; it, him her, them; go to
This function of 之 is different from the one in your quoted text.
之 in [Verb + 之] functions the same as a pronoun
Example: "逆我者，殺(之)" = "the ones who against me, kill (them)"
In this sentence, the verb 殺 precedes 之, therefore, 之 functions as the pronoun ...
Four of the five "Big Five Profanity Words of Guangdong (廣東粗口五大字)" can be used as a superlative before an adjective. The one that couldn't is '屌', which is strictly a verb for 'f..k'
To use a swearing word as a superlative, it must be placed after the superlative adverb and before the adjective, not inserted between the two characters of the ...