Similarities:
- Based off of similar animals 
 
Crayfish and crabs are both arthropods which live near water 
- Both look like they have a fighting prowess
 
Crabrawler looks it could fight with those boxing gloves, while Crawdaunt gives off an intimidating appearance with its big pincers and wide eyes
Stand upright on legs with two pincers in the air like hands
- Similarly structured names 
 
Prefix is Cra; suffix pertains to their typing:
Brawler is a take on "brawl", which is defined as a noisy fight, hence the Fighting type 
Daunt is defined as making someone feel intimidated, a characteristic of Dark types 
Even with all of this, there isn't really anything to show that they are directly related in any way; more likely they coincidentally ended up being similar, like Arcanine and Entei, who are both Fire types, have a dog-like appearance and have similar stats but don't have any connection.