It was originally supposed to be a cow/ox, the cow was made & filmed but the studio then decided against it as the Xenomorph would have been too bulky and slow. That's the reason it was then changed to a dog (faster)

The whole idea was a mistake in my eyes as the CGI took a lot away from the film for me. 

*Alien 3* as a whole was a great concept 'Prison with no weapons etc' just poorly executed.