The forms ipse, ille, and iste look like vocatives instead of nominatives, and one would expect to see ipsus, illus, and istus instead. In fact, ipsus is attested but far rarer than ipse, but the other two forms seem to be entirely absent. Is there a reason for this unusual morphology? Is it related to the vocative (perhaps having a similar origin) or is the similarity coincidental? Is there something analogous in other languages or with other pronouns, or anything else that might shed light on these unexpected forms?
Some pronoun singular nominatives are quite wild (is, qui, illud, …), but the three I brought up are only mildly weird, and therefore I hope that an explanation actually exists. If I remember correctly, the singular masculine vocative of the second declension is obtained from the pure stem (without the nominative ending -s), with a vowel change o>e. Any evidence for tendency to drop the -s for pronouns in singular masculine nominative would be interesting if that is the case.
It appears that ipsus was more popular in pre-classical Latin, and kkm remarks in a comment that olle and ollus are attested for ille in old Latin. This gives the impression that the expected -us ending was there but was later lost. Any insight as to why or how this happened would be great.
