Every Monday we will post an entry that hasn’t yet been published with a view towards harnessing the collective onomastic power of the internet. If you have any thoughts about the name’s origin, other variants it might be related to, other examples of its use, etc., please share them in the comments! If you wish to browse other Mystery Monday names, there is an index.
Here’s one for the Breton experts!
The prototheme is Old Breton hael ‘generous, noble’, a relatively common theme found in the names Brithael, Haelcar, Haelhoiarn, Haelnou, Haeloc, and Haelwaloe, but we don’t recognize the deuterotheme. If you have any thoughts of what it might be, please leave a comment!
Dictionary of Medieval Names from European Sources 
Have you considered the possibility that it might be dittographic for Hailgur or the like? Léon Maitre & Paul de Berthou, Cartulaire de l’abbaye de Sainte-Croix de Quimperlé, 2nd ed., 1904, is online here, and on p. 145 there is an entry for a Haelguri. Gur- ‘warrior, man’ is a common enough prototheme, but I don’t know how well attested it is as a deuterotheme.
The name appears as “Hailgugar” in an earlier transcription of that document by the same scholar (A. de Courson, “Histoire des peuples Bretons…” I, 1846, p. 394). In that case the deuterotheme might be -car “friend, lover” which is certainly common. It depends on which one is right. If it’s “-gar”, the name might be the same as “Haelhucar”, “Haelhocar” attested as the name of an abbot in Brittany at the time (analyzed by Loth, “Vocabulaire Vieux-Breton” p. 150 as hael + hu-car “bien-veillant, aimant”). See the discussion in this paper:
http://www.asnc.cam.ac.uk/conversion/logos/Flechner_Hibernensis.pdf
This would make it the same as “Haelocar” that you already have – see note 27 in that paper: ‘the name “Haelhocar” occurs only in two charters of the Redon Cartulary …. The name in § 8 is wrongly transcribed in the edition as “Haelocar”. The facsimile of the charter clearly reads “Haelhocar”.’