Ver es creer (“Seeing is believing”).
In Old Spanish — the Spanish of El Cid, spoken into the 1400s — ver and creer had more in common than this refrán. The Old Spanish version of ver was veer, and its conjugation was identical to that of creer (and likewise leer).
The three verbs were similar because they had similar Latin roots: videre, credere, and legere. In all three cases, the middle consonant (d or g) dropped out, creating a double ee in all but the yo form. (The change of i to e in videre was normal, see this earlier post.)
Although I didn’t bother to include it in the table above, poseer, from Latin possidēre, fits the same pattern.
Veer became an irregular verb when its history diverged from creer and leer. As Old Spanish gave way to the modern language, the double ee of veer simplified to a single e. (The expected yo form from this point onward was vo). Later, the –edes ending for vosotros changed to –eis for all -er verbs. These two changes together gave rise to the modern conjugations:
Ver‘s history is both unique and familiar. Unique, because it created the only verb with this specific irregularity. (I’m not counting prever, which is of course based on ver.) Familiar, because as with the much more frequent -zco and -go verbs, the more conservative yo form, ironically, became irregular as a side-effect of changes that affected other forms of the verb, including the infinitive. Veo itself would have remained just as regular as creo and leo if the other verb forms hadn’t changed.
So, dear reader: Viste. Leíste. ¿Creíste?