Here is a selection of Ultra Monster illustrations done by Takayoshi Mizuki between 1965 and 1972.

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraseven vs. Aron

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraman vs. Gabula & Rigger

Ultra Kaiju illustration by Takayoshi Mizuki --
Alien Goron vs. Gumonga

Ultra Kaiju illustration by Takayoshi Mizuki --
Kaiju Ebanashi

Ultra Kaiju illustration by Takayoshi Mizuki --
Alien Godola, Alien Borg, Alien Baltan, Alien Zarab, Alien Icarus, Alien Bado

Ultra Kaiju illustration by Takayoshi Mizuki --
Eleking, Dino-Tank

Ultra Kaiju illustration by Takayoshi Mizuki --

Ultra Kaiju illustration by Takayoshi Mizuki --
Capsule Monsters: Windam, Agira, Miclas

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraseven

Ultra Kaiju illustration by Takayoshi Mizuki --

Ultra Kaiju illustration by Takayoshi Mizuki --

Ultra Kaiju illustration by Takayoshi Mizuki -- Ultra Kaiju illustration by Takayoshi Mizuki --
Alien Baltan // Magular

Ultra Kaiju illustration by Takayoshi Mizuki -- Ultra Kaiju illustration by Takayoshi Mizuki --
Garamon // Kanegon

Ultra Kaiju illustration by Takayoshi Mizuki -- Ultra Kaiju illustration by Takayoshi Mizuki --
Litra & Gomess // Peter & Todora

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraseven, Alien Icarus, Alien Pega, Alien Cool

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraseven, Bostang, Starbem Gyeron

Ultra Kaiju illustration by Takayoshi Mizuki --
Kaiju Ebanashi

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraman vs. Jirass

Ultra Kaiju illustration by Takayoshi Mizuki --
Seabozu & Zaragas

Ultra Kaiju illustration by Takayoshi Mizuki --
Peguila & Kanegon

Ultra Kaiju illustration by Takayoshi Mizuki --
Jamila & Alien Mephilas

Ultra Kaiju illustration by Takayoshi Mizuki --
Kaiju Ebanashi

Ultra Kaiju illustration by Takayoshi Mizuki --
Aboras, Banila, Zambolar

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraman vs. Jamila

Ultra Kaiju illustration by Takayoshi Mizuki --
Ultraman, Alien Baltan, Bemular

[Link: Takayoshi Mizuki]