Serbia

Serbi are mentioned by Pliny the Elder as Scytian people taking place between Montes Ceraunii and the river Rhea.
Pliny the Elder lived between 0 AD and 100 AD when this was recorded.
As to the name of Agathodaemon which is a snake and name of Serbia which is Serpia (B and P are interchangable) it is logical that ancient Serbi were still holding to the oldest belief on Earth and worshipped Snakes as Holly Deity.
Later on in time the Snake became demonised, in Christianity. 

Excerpt from the Byzantian cronicle states that

during visit to Emperor Vasilis 

Basil II "the Bulgar-Slayer"

(Βασίλειος Β΄ ὁ Βουλγαροκτόνος)

Scytian delegation

composed of Croatian and Serbian representatives

demanded to become subjects to the Byzant

after terrible raids of Romans in Dalmatia.

source;

Skylitzes Chronicle

original transcript and translation on the pictures.

The Slovaks of Hungary-Slavs and Panslavism,

Thomas Capek,Knickerbocker Press, 1906

The Slovaks of Hungary- Slavs and Panslavism,
Thomas Capek, Knickerbocker Press, 1906

Turcogreacia libri 8 - Crusius , Martin
Basilea 1584 ,page 342


Tribali Bulgari are Serbian people precisely

Triballi , alijs Bulgari putantur : fed Nicephoro Gregore Servij funt :
quia libro eius 9.cap.15 Michael Bulgariae Rex , a Crale (Rege) Serviorum



from Oxford University Press
A book published in 2014 from historian Clifford R. Backman states that Slavs didn't displace indigenous Croats, Srbs and Macedonians but that the Avars adopted the language that was already spoken on this areas.
In short Clifford R. Backman does not dispute Illyrian herritage of named population but he combines DNA and historical evidence. As Croats, Srbs and Macedonians have indigenous I haploup group in large percentage it is certain that these people are ancient Illyrians and that Scythian tribe of Avars (not Mongolian but white mostly redish haired population arrived from Asia which was entirely Scythian meaning white, blond or redhaired) after Mongolian invasion on Xiongnu Emporium which was Scythian or Amazon in the very early begin.  Xiongnu Emporium fell around 400 AD and Avars and Huns moved westwards towards Russia and Europe in those times.
They, Avars would be that R1a component in Southern Slavic gene pool.
It is not impossible that Avars already spoke form of Slavic because Tocharians (Xiongnu neighbours) spoke very similar language to Slavic.
 (numbers are identical to slavic also kcer is cker in Tocharian and much more)
Tocharians too were redhaired or blond, there was no Mongolian mummies in Asia this old.

Croatia

Acording to Pliny the Elder ,ancient writer Arvati have been at the same place where they are today between 0ad and 100ad.
There is no mention of their arrival, they have been there amongst other ancient tribes.
Most interesting is that inhabitants of  Dalmatia ,well known Illyrian tribe
called them up to these days Arvati.

Records on continuance of Croatian nation exist in Harvard files online.
Left side is in latin and right side translated in english.

Croatia Rubea during Diocletian in Dioclea (around 300-400 AD)

from;
Topography of
Big Hungarian Kingdom
18th century Wienna.

Excerpt from the Byzantian cronicle states that
during visit to Emperor Vasilis
Basil II "the Bulgar-Slayer"
(Βασίλειος Β΄ ὁ Βουλγαροκτόνος)
Scytian delegation
composed of Croatian and Serbian representatives
demanded to become subjects to the Byzant
after terrible raids of Romans in Dalmatia.
source;
Skylitzes Chronicle

original transcript and translation on the pictures.

The Slovaks of Hungary-Slavs and Panslavism,

Thomas Capek,Knickerbocker Press, 1906

"Scythians and Greeks"

Ellis Hovell Minns

Getae were also Carpi or Carpiani (Ptolomei) between the Tyras and Ister with town Harpis on the coast. Niederle suggestes they were Slavs, the same as the enigmatical Khorvate or Croats.

from Oxford University Press
A book published in 2014 from historian Clifford R. Backman states that Slavs didn't displace indigenous Croats, Srbs and Macedonians but that the Avars adopted the language that was already spoken on this areas.
In short Clifford R. Backman does not dispute Illyrian herritage of named population but he combines DNA and historical evidence. As Croats, Srbs and Macedonians have indigenous I haploup group in large percentage it is certain that these people are ancient Illyrians and that Scythian tribe of Avars (not Mongolian but white mostly redish haired population arrived from Asia which was entirely Scythian meaning white, blond or redhaired) after Mongolian invasion on Xiongnu Emporium which was Scythian or Amazon in the very early begin.  Xiongnu Emporium fell around 400 AD and Avars and Huns moved westwards towards Russia and Europe in those times.
They, Avars would be that R1a component in Southern Slavic gene pool.
It is not impossible that Avars already spoke form of Slavic because Tocharians (Xiongnu neighbours) spoke very similar language to Slavic.
 (numbers are identical to slavic also kcer is cker in Tocharian and much more)
Tocharians too were redhaired or blond, there was no Mongolian mummies in Asia this old.