Although Sunquan does an amazing job with his videos, it is certainly possible he made an error. Plus, as Qvaak points out, there is some ambiguity in how words get truncated to create inflected forms. The epenthetic /e/ is not entirely irregular. A large number of cases where it is added are covered by a rule on (I think) page 56 of David's book (this needs to be added to the wiki, but I haven't had time lately), but even that isn't entirely certain. If unsure, check the wiki. All known irregular stems are given there, and this will be coming soon to the dictionary, as well. This /e/, in most cases, doesn't hurt things. If you think it needs to be there, it usually doesn't hurt to use it. Although development on Dothraki has slowed a lot, it hasn't stopped, and we try and keep everything here as up to date as we can.