There was the agent suffix -ter- of PIE. It was used for creation of terms for relatives and for creation of agent nouns.
Suffix -ter- was used to create a noun for person whose function or profession was to perform the action (irrespective whether he actually did it) while the o-grade of it -tor- was used to denote a person who just did the action. The combination of suffixes -a̯-ter- was used for some relatives.
Thus the term for daughter in PIE was dhuga̯tēr.
The same suffix in zero-grade, "-tr-" in combination with inanimate ending "-om" was used for creating words for tools, such as a̯ero̯trom "plow", u̯estrom "wear", tere̯trom "auger", costrom "knife".
Thus regular PIE rules suggest that a combination of the root for laugh, "kleg-" plus "-tr-" such as in "klegtrom" should give a non-animate noun related to laugh. In Pre-Germanic the ending could change from inanimate -om to masculine -os to render (via metathesis) *hlahtraz in Proto-Germanic.
It is more possible though that the word by this same process was formed already after Germanic branch split of PIE because other branches do not show use of the -tr- suffix with this root.
By the way, of course, the PIE superlative -ter-os was also another use of the same suffix. Thus "gela̯teros" would mean "more joyful". The word "enter" (from Latin intra-) is another example of use of this suffix with adverbs: e̯en "in" + "-ter-om" -> e̯enterom "intestines", of which locative case is e̯enteri "inside".