"Hyundai" is just a romanisation in an older romanisation system. Over the years, there have been many different systems for the romanisation of Korean - over 20. The current official system is the Revised Romanization of Korean, adopted by the Korean government in 2000. This was preceded by the McCune-Reischauer romanisation system, which was developed in 1937, and adopted by the Korean government in 1948. Before that, different romanisation systems were in use; the most common one in use by English speakers was Gale's system of 1897. Most Korean names with non-McCune-Reischauer/non-Revised Romanization romanisations use some version of Gale's romanisation.

Gale's romanisation simply treats the vowel ㅐ as ㅏ + |, with a romanisation of ai = a + i. This romanisation of ㅐ was also used by some other romanisation systems. Gale romanised ㅕas yö or yü, and dropping umlauts these become yo and yu. (Dropping diacritics like this is common, and is one of the motivations for the development of the Revised Romanization which doesn't use them - McCune-Reischauer with diacritics dropped is ambiguous.) Thus, 현대 is Hyundai in a no-diacritic Gale romanisation. Note that the founder of Hyundai, 정주영, used the Gale romanisation of Chung Ju-yung for his name (Jeong Ju-yeong in Revised Romanization and Chŏng Chuyŏng in McCune-Reischauer).

A comparative table of the Gale, McCune-Reischauer, and Revised Romanization romanisations is given below. The Gale romanisation is from Gale (1897), with the "+ |" rule noted above used for the vowels not appearing in Gale's table. Note that these romanisations are not transliterations. That is, there isn't a one-to-one mapping of Hangul vowels and consonants to the romanisation. These romanisations attempt to represent the pronunciation, and different romanisations can be used for the same consonants (where two romanisations are given for McCune-Reischauer and Revised Romanization below, these are for consonants in the initial and final positions in the Hangul syllable).

Hangul	Gale	MR	RR
ㅏ	a	a	a
ㅐ	ai	ae	ae
ㅑ	ya	ya	ya
ㅒ	yai	yae	yae
ㅓ	ö,ü	ŏ	eo
ㅔ	öi,üi	e	e
ㅕ	yö,yü	yŏ	yeo
ㅖ	yöi,yüi	ye	ye
ㅗ	o	o	o
ㅘ	oa	wa	wa
ㅙ	oai	wae	wae
ㅚ	oi	oe	oe
ㅛ	yo	yo	yo
ㅜ	u	u	u
ㅝ	uö,uü	wŏ
ㅞ	uöi,uüi	we	we
ㅟ	ui	wi	wi
ㅠ	yu	yu	yu
ㅡ	eu	ŭ	eu
ㅢ	eui	ŭi	ui
ㅣ	i	i	i
ㄱ	k,g	k	g,k
ㄲ	kk,gg	kk,k	kk,k
ㄴ	n,l	n	n
ㄷ	t,d	t	d,t
ㄸ	tt,dd	tt	tt
ㄹ	l,n	r,l	r,l
ㅁ	m	m	m
ㅂ	p,b	p	b,p
ㅃ	pp,bb	pp	pp
ㅅ	s,t	s,t	s,t
ㅆ	ss,tt	ss,t	ss,t
ㅇ	ng	ng	ng
ㅈ	ch,j	ch,t	j,t
ㅉ	jj	tch	jj
ㅊ	ch'	ch',t	ch,t
ㅋ	k'	k',k	k
ㅌ	t'	t',t	t
ㅍ	p'	p',p	p
ㅎ	h	h	h,t

The history of official romanisations in Korea between 1959 and 1984 is somewhat convoluted, since the Ministry of Education adopted a romanisation that was a transliteration in 1959, and different government agencies used different romanisations until 1984 when the government adopted a modifed version of McCune-Reischauer for everybody; this was used until the adoption of Revised Romanization.

With the adoption of Revised Romanization, the government allowed people and companies to continue using their traditional romanised forms, but strongly encouraged new companies to follow Revised Romanization, and would "gladly welcome decisions by companies using inconsistent names to follow the new system". Noises are sometimes made about enforcing names (people and companies) to use Revised Romanization, which generally gets strong push-back from those who would be affected.

Reference:

James S. Gale, A Korean-English dictionary, Kelly & Walsh, 1897. https://archive.org/details/KoreanEnglishDictionary/