Dragonoid is a pretty utilitarian term, it's made up of the word dragon with the suffix -oid which means "like" or "resembling" (think humanoid/anthropoid which both mean non-human with human form/characteristics, ovoid which means "resembling an egg" or "egg shaped", etc.). Thus, dragonoid means something that "resembles a dragon" or is "dragon-like" without being a proper dragon. The Japanese term for dragonoid shared between DQ's dragon knight's dragonoid form and Slime's Milim's race is 竜魔人 (romanji: ryūmajin, lit: dragon monster person) is likewise quite utilitarian but each for different reasons.
In DQ the 竜の騎士 (romanji: Ryū no kishi lit: dragon knight) race was created from the collaboration between the god of dragons, god of darklings and god of humans which is reflected in the name of the dragonoid form which takes a character from each. However, do note that dragonoid is not the name of the race but of a form. "Dragonoid form" doesn't seem to carry any history beyond DQ and as a name of a form is usually utilitarian (demon form, final form, super (saiyan), etc.) and somewhat arbitrary.
On the other hand, Milim's race is 竜魔人 (dragonoid), birthed from the union of a human and dragon. In most media her race would be called "half-dragon" but slime adds the "monster" bit because in its universe all beings borne from magicules (i.e. mana) are considered monsters and they specifically define the term 魔人 (romanji: majin lit: monster person) to denote humanoid monsters. Thus she is a humanoid monster of the draconic variety. The half dragon race of beings carries a more storied history. Chinese myth believes that the emperors are descended from dragons which necessarily means dragons can make "the beast with two backs" with humans. A hulking lizard getting busy with a tiny human... how does that work? Another core trait of dragons is their ability to shapeshift which is rooted in antiquity from stories of humans being transformed into dragons Cadmus in Greek myth and Fafnir in Norse myth being the most famous inspirations. In more modern times we have the classic D&D origin story: a dragon + a human bard + nat 20 roll in Charisma = half-dragon babies. Given that we're talking about anime we should also consider dragonoids from the opposite direction too, rather than being rooted in the largely western idea of a "half-dragon" we think of Milim as a "dragon girl" which is a natural extension of "cat girls". A mainstay of otaku culture, cat girls probably trace lineage to folklore of Kitsune (fox yokai) who've long been believed to be able to shapeshift into beautiful women as well as stories of Bakeneko (cat yokai) prostitutes which were prevalent in the Edo period. According to Wikipedia the first anime to feature cat girls was "The King's Tails" made in 1949. Now a ubiquitous trope in anime and manga cat girls are the poster child of kemonomimi (human-like beings with animal ears) at large which includes raccoon girls, horse-girl idols and even Isis-chan.
Bakugan's Dragonoid seems mostly like a one-off name, Dragonoid seems to exclusively be written in katakana (ドラゴノイド romanji: doragonoido) which means its intended to be a loan word from the English word "dragonoid" rather than a choice of reading for 竜魔人 like the other two. Bakugan also makes abundant use of the -oid suffix in their names: Serpenoid, Centipoid, Hydranoid, Ravenoid, etc. Likely because naming things by corrupting existing English words sounds cooler (dragonoid > dragon, ravenoid > raven, etc.) and improves SEO.
So where does the idea of dragonoids come from? Likely nowhere, the three given examples seem to be the only uses of dragonoid, have a mixed bag of influences and none of them have anything to do with each other. Rather than being inspired by each other it seems as though 3 separate franchises independently saw the utility the name dragonoid