Animate Dead - spell effects

I just cast the lvl.3 cleric spell Animate Dead and summoned... a ghast ! I do not remember this happening in BG2, that undead creature is sure a lot more powerful than a skeleton. This could really have a big effect on several battles in BG1.

Does anyone know if it is always a ghast that is summoned? or can you get a skeleton, zombie, ghoul...?