Evolution Synchro Monsters are a series of Synchro Monsters that require an upgraded version or special number of Tuner Monsters.
Currently, the only 6 known Evolution Synchro Monsters in the TCG and OCG are "Shooting Star Dragon", "Red Nova Dragon", "Vairon Omega", "Tech Genus Halberd Cannon", "Tech Genus Blade Blaster", and “Shooting Quasar Dragon”.
They are not Synchro Summoned in the usual way. Shooting Star Dragon, Shooting Quasar Dragon, Tech Genus Halberd Cannon and Tech Genus Blade Blaster require a Synchro Tuner Monster and Synchro Monsters as the Synchro Material Monsters (called an Accel Synchro Summon in the anime). In contrast, Red Nova Dragon and Vairon Omega require Red Dragon Archfiend and a Vairon monster, respectively, and 2 Tuner monsters to Synchro Summon it (called Double Tuning in the anime). Though Red Nova Dragon and Varion Omega are currently the only Synchro Monsters that specifically say that 2 Tuner Monsters are needed, Double Tuning is possible with XX-Saber Gottoms when the "EARTH" monster(s) is also a Tuner Monster, but also Gottoms can be Summoned with any number of Tuner Monsters, provided they are EARTH.
It also should be noted that both Red Nova Dragon and Shooting Star Dragon share the effect: "When an opponent's monster declares an attack, you can remove from play this card to negate the attack. If you negate an attack this way, Special Summon this card during the End Phase." Additionally, the ATK of each of Shooting Star Dragon and Red Nova Dragon is 500 points less than their Majestic forms, and their DEF is equivalent to their original form's ATK.
On Yu-Gi-Oh! Card Maker Wikia, however, Evolution Synchro Monsters encompass several subsets, such as Limit Over, Delta, and regular Accel or Hyper Synchros, Double Tunings, Burst Synchro Monsters, Vortex Synchro Monsters, and Exceed Synchro Monsters.