51+ animals not summoning sounds correct to me.
Here is an example of a Velious animal mob who is always lvl 52.
According to numerous posts in this thread, DOES NOT summon.
http://eqbeastiary.allakhazam.com/search.shtml?id=6145
An example of some mobs 51+ summoning and some not are the neriads in Siren's Grotto
All of them are above level 51.
Of the non-named, only the "a neriad weaver" will summon.
Interestingly the "a neriad weaver" spawns as lvl 60-61 all others are mid 50s
I spent countless hours in SG farming the drum and shawl.
Shimmering Sea Spirit
Here is an example of a 51+ "named" animal mob who DOES NOT SUMMON
He is a rare spawn from the other water elementals in zone, has 2 peices of loot so I would consider him a "named"
Fellspine
Here is an example of a 51+ "named" animal mob who DOES SUMMON