I'm not sure if this is a bug, or intentional. But it used to be that when one approaches a trapped skeleton horse and trigger the trap, the inital "trap" skeleton horse would have the highest health value possible in Vanilla, 53 HP, and the 3 other Skeleton Horses would have normal, randomly distributed values.

Since a 1.11 snapshot, all 4 skeleton horses only have 15HP (displayed as 7 hearts), including the initial "trap".

From my own experiences in 1.10.2, I know that the main horse (with the SkeletonTrap) always has 26 hearts (53 healthpoints/HP), and the other 3 got lower values, but overall well distributed, like normal living horses, and not all just mere 7 hearts (15 HP).

The skellyhorses I tested were all spawned naturally by the game in 16w43a (lightning/thunderstorm), but I also tried the command-approach and there you get the very same result:

Steps to reproduce
1. (Optional) Set the difficulty to "peaceful" to get rid of the skellyriders immediately upon summoning

2. Insert this command:

/summon minecraft:skeleton_horse ~ ~ ~ {SkeletonTrap:1b}

3. Sit on all 4 skellyhorses, they should all show just 7 hearts.

The 7 hearts/15 HP are not only the constant value for when the skeletontrap-skellyhorse is being summoned via command and due to natural spawns with a lightningbolt, but it also is the case when spawning them with a skeleton horse spawn egg.

If you spawn living horses with a spawn egg, their values are random, not so for the skeleton horses as well as zombie horses though, they all only show only 7 hearts but it should be random with a spawn egg, to be consistent with the regular horses, and even more so as you can now (since 18w19a) ride Skeleton Horses underwater so a higher health would be crucial, e.g. in a fight with Undead underwater, if you're riding a skellyhorse.

If you fix this for skeleton horses, please fix it for zombie horses as well.