This question is best illustrated in an abridged example:
Combat has already begun and a goblin sees a Paladin on his noble steed. He readies an attack on this trigger: When the Paladin enters my range, I will attack him.
The Paladin's turn begins, and he readies an attack as well - When the Goblin enters my range, I will attack him.
The Paladin's mount then goes, rushing down towards the Goblin, Paladin on his back. He enter's the Goblin's range, and...
Who strikes first?
RAW, I couldn't find a definitive source that solved the matter - I wanted to say that because the Goblin had a higher initiative count, his reaction went first. However, the Paladin has quite impressive Dex, and argued that it should be a contest of Sleight of Hand, which also makes sense to me.
Initiative has a small section about resolving standard ties with initiative, but I feel it doesn't apply here after initiative has already been determined.
"...The GM can decide the order if the tie is between a monster and a player character. Optionally, the GM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first."
I am also aware of this question, which provides an answer, but using 3.5 Edition Rules.
Who goes first in a simultaneous reaction trigger when combat has already begun in 5th edition? Is there any clarification in the rules from 5th Edition sources?