Tex is a robot character. Although he did not appear in a Pixar feature or short film, he was created by John Lasseter for Lucasfilm's THX sound system, which partnered with Pixar films from 1995 to 2007. He appeared in three THX trailers. The first was released in 1995 (alongside Toy Story). The second and third promos were released in 2001 and 2006, respectively. While he does not have any comprehensible dialogue, his voice was provided by Lasseter.
Tex flying around the THX logo in the first trailer
Tex in front of the THX logo in the first trailer
Tex with a moo can in front of the THX logo in the second trailer